Defender of the Crown
Sold on GOG for $5.99. A few weeks after the GOG release, a collection of Cinemaware games featuring this plus 12 other games showed up on another platform for a mere $9.99.
Wings (Emulated Amiga Edition)
Same as above.
Rocket Ranger (Emulated Amiga Edition)
Same as above.
Botanicula
GOG offered an exclusive pre-order deal for this game. Come release day, Botanicula was featured in a Humble Bundle at the "pay what you want"-base tier with a slew of extra content not present in the GOG version.
Clustertruck
The gog version is missing the level editor, replays, leaderboards, and the ability to play custom maps. Both the editor and replays were initially advertised on the game's store page, but have been quietly redacted.
Perception
Features a game-breaking bug in the final chapter. Devs refuse to patch the gog version because they "have issues with gog" (they don't have issues taking money from gog customers for a broken game, though). Missing content update that adds new modes
Space Colony HD
"They made an updated definitive version exclusive to Steam. It not only includes the usual Steam nonsense, but also adds HD graphics upgrade, a re-balance to easy mode, and a new mini campaign." (micktiegs_8)
Escape Goat 2
No level editor on gog
Shadwen
Shadwen's level editor is Steam-exclusive. The developer mentioned that they have no plans to change this, as they have linked the sharing with the Steam workshop feature.
Deathtrap
This DRM-Free version does not contain the Level Editor, Workshop and Custom Map support
Overfall
Is apparently missing the "Story Creator Mode"
Necropolis: Brutal Edition
Multiplayer/Coop is missing from the GOG version despite it being advertised during the pre-order campaign. Harebrained Schemes promised to add these features at a later point, but instead decided to drop further development of this game altogether.
Manhole: Masterpiece Edition, The
The GOG version of The Manhole: Masterpiece Edition costs $5.99, while the publisher offers the Cyan Children's Pack on Steam, which contains The Manhole in addition to two other games for the exact same price.
Luftrausers
A bug prevents the completion of this game. "The fix came out just days after the game was released... on Steam. The devs have not made the effort to release a full update to all platforms as they told me they were doing." (ForsakenEagle)
Tales of Monkey Island
Horrible sound quality in second episode. This is a very old problem and TellTale team confirmed that they fix it and updated game on their site and on Steam... over three years ago! (pykman)
The Dark Eye: Chains of Satinav
Missing the original German language audio. According to Daedalic, this was due to the flat pricing. However, the games are regionally priced now and still the GOG version does not feature the German audio. Mails to Daedalic have gone ignored.
Memoria
Same as above.
Guacamelee! Super Turbo Championship ...
A patch was supposed to add 4 Player co-op support and additional fixes. gog user Irenicus73 e-mailed the developers about this update, and got this reply back: "chances are the GoG version won't be getting the patch. cheers chrisM"
Wings!™ Remastered Edition
The GOG version of Wings! Remastered seems to have received a flawed version of the latest patch that removed the voice-overs.
Brutal Legend
No multiplayer in the DRM-free build. At least they're upfront about it and explicitly mention that on the game page.
Ironclad Tactics Deluxe Edition
The DRM free version of Ironclad Tactics Deluxe Edition is single-player only.
The Chaos Engine
The GOG version has no online multiplayer, while a version sold somewhere else does. (Grargar)
Albedo: Eyes from Outer Space
The GOG version has not changed since April 2015 and is missing the May 2015, June 2015 and September 2015 updates available on Steam.
Lovely Planet
The GOG version of Lovely Planet doesn't feature leaderboards. Email I got from the developer calls it "inferior". (hyperagathon)
OlliOlli
Missing daily challenges (Grargar) and leaderboards (sh4dow83)
OlliOlli2: Welcome to Olliwood
Same as above.
Spelunky
No daily challenge in the DRM free version (DAlancole)
Outlast
GOG version is missing the soundtrack that is featured on the version sold on another platform.
Nuclear Throne
This game was released on December 5th, 2015. The GOG version has yet to receive a single patch
Samorost 2
Amanita Design decided to screw over Day 1 gog buyers by selling this game at $0.99 on Humble literally one day day after releasing it on gog.
Hatoful Boyfriend
On other stores, a Collector's Edition which offers a variety of extra can be purchased, but there is no option for it here.
STAR WARS™ Knights of the Old Repub...
As of yet, this didn't receive the 2015 patch released by Aspyr, despite Aspyr reassuring gog users that it is being worked on as far back as February 2016.
Starbound
The dev/pub, has chosen to block third-party downloads from Steam workshop, making it impossible to get workshop-exclusive mods mods if you are not a non-Steam user. Also, there is no option to purchase the soundtrack, as you can do on Steam.
Mirror's Edge™
The Time Trials DLC pack is Origin-exclusive.
Offworld Trading Company
According to the devs, the gog version does not have and won't have the whole multiplayer side implemented. There is no way to play with friends on gog. No custom lobbies either. No word about this when the game launched, cut features were advertised
DLC: Offworld Trading Company - Alman...
See above.
DLC: Offworld Trading Company - Ceres...
See above.
DLC: Offworld Trading Company - Scena...
See above.
DLC: Offworld Trading Company - OST
See above.
DLC: Offworld Trading Company - Real ...
See above.
DLC: Offworld Trading Company - The P...
See above.
Punch Club
The last patch for Punch club introduces easy mode and GOG version still suffers from a bug which was fixed last March.
Punch Club Deluxe Edition
The last patch for Punch club introduces easy mode and GOG version still suffers from a bug which was fixed last March.
Worms W.M.D
Current build is several months out of date and is missing many big updates
Aragami
No level editor, because it's "heavily linked to the Steam Workshop API. An effort could be done to detach it, but we would need to redesign the user interface and do some testing. In any case, it's a lot of effort to implement a half-baked solution"
Party Hard
Gog version is missing the level editor as well as the "High Crimes" DLC.
Ball, The
The GOG version is missing three Suicidal levels (basically Challenge levels) incl. a Portal themed one.
Serious Sam: The First Encounter
Serious Sam Classics: Revolution is free on Steam for owners of either Serious Sam: The First Encounter or The Second Encounter while it isn't even available on gog.
Serious Sam: The Second Encounter
Serious Sam Classics: Revolution is free on Steam for owners of either Serious Sam: The First Encounter or The Second Encounter while it isn't even available on gog.
DLC: Crypt of the NecroDancer: AMPLIFIED
Missing four soundtrack DLCs available on Steam.
Turmoil
At the request of the publisher gog region locked this game in China, where it is still readily available on Steam.
Doom 3: BFG Edition
No multiplayer in the gog version (however, it can be enabled via console commands). Due to censorship and various gameplay changes, the BFG Edition is generally considered inferior to the original Doom 3, which is not available as of yet.
Mother Russia Bleeds
No leaderboards in the gog version.
Mother Russia Bleeds: Dealer Edition
No leaderboards in the gog version.
DLC: Botanicula Soundtrack + Art Book
Oh, guess what: The Humble-exclusive bonus content finally showed up here, 5 years after the fact! Unfortunately, people who who fell for Anamita's EXCLUSIVE pre-order deal on gog won't be getting this upgrade for free.
Arx Fatalis
Missing French, German, Italian, Spanish and Russian versions found on Steam
SpellForce Platinum
Missing German and French versions available on Steam
Sacrifice
Missing French, German, Italian and Spanish versions found on Steam
Pajama Sam Vol. 1
Missing German, Dutch and French versions of Pajama Sam 1&2, all of which are on Steam.
Pajama Sam Vol. 2
Missing Dutch and French versions of Pajama Sam 3, which can be found on Steam.
Majesty Gold HD
Apparently got update with modding support on Steam, but never on GOG.
AquaNox
Missing German, Italian and Spanish version
AquaNox 2: Revelation
Missing German and French version
Cossacks Anthology
Missing French, German and Russian version
Earth 2140 Trilogy
Missing Italian and German version
Earth 2160
Missing French, German, Spanish, Italian, Czech and Polish version
Gorky 17
Missing French, German, Spanish, Russian and Polish version
Railroad Tycoon 3
Missing French, Italian, German and Spanish version
Vampire®: The Masquerade - Bloodline...
Missing German and French version
Army Men
Not for sale in South Africa and South Korea. This was supposed to be temporary, but the lock has not been lifted, despite Steam having released this game in the meantime without this region lock.
Army Men II
Not for sale in South Africa and South Korea. This was supposed to be temporary, but the lock has not been lifted, despite Steam having released this game in the meantime without this region lock.
Army Men RTS
Not for sale in South Africa and South Korea. This was supposed to be temporary, but the lock has not been lifted, despite Steam having released this game in the meantime without this region lock.
Army Men: Toys In Space
Not for sale in South Africa and South Korea. This was supposed to be temporary, but the lock has not been lifted, despite Steam having released this game in the meantime without this region lock.
Hidden & Dangerous Action Pack
Not for sale in South Africa and South Korea. This was supposed to be temporary, but the lock has not been lifted, despite Steam having released this game in the meantime without this region lock.
Hidden & Dangerous 2: Courage Under Fire
Not for sale in South Africa and South Korea. This was supposed to be temporary, but the lock has not been lifted, despite Steam having released this game in the meantime without this region lock.
Hyperdimension Neptunia Re;Birth1
Not for sale in Japan, while it is readily available on the Japanese Steam storefront. Gog version does not feature Japanese text. No option for gog customers to purchase the Deluxe Edition, which features, among other things, the soundtrack.
DLC: Hyperdimension Neptunia Re;Birth...
Not for sale in Japan, while it is readily available on the Japanese Steam storefront.
Hyperdimension Neptunia Re;Birth2: Si...
Not for sale in Japan, while it is readily available on the Japanese Steam storefront. Gog version does not feature Japanese text. No option for gog customers to purchase the Deluxe Edition, which features, among other things, the soundtrack.
DLC: Hyperdimension Neptunia Re;Birth...
Not for sale in Japan, while it is readily available on the Japanese Steam storefront.
Hyperdimension Neptunia Re;Birth3 V G...
Not for sale in Japan, while it is readily available on the Japanese Steam storefront. Gog version does not feature Japanese text. No option for gog customers to purchase the Deluxe Edition, which features, among other things, the soundtrack.
DLC: Hyperdimension Neptunia Re;Birth...
Not for sale in Japan, while it is readily available on the Japanese Steam storefront.
Dragon's Dogma: Dark Arisen
Region locked in Japan on gog, not on Steam. Missing two updates dating back to October and December 2017.
Sanitarium
Missing German and French language version
BIT.TRIP Presents... Runner2: Future ...
Missing DLC: "Runner2 - Good Friends Character Pack"
SteamWorld Heist
Hasn't been updated since it has been released here. Missing a significant update (released in September '17) that fixed save issues.
Bard's Tale, The
Was updated to Remastered and Resnarkled Edition on Steam more than two months ago. These are the developer's words on the matter: "We'll be bringing it to GOG eventually, vv."
Door Kickers
Multiplayer is currently Steam only because the dev lacks the resources to implement the Galaxy SDK here. Out of date since November '17 (at the very least - last update on gog in May '17)
Death to Spies
Missing the French version, which is included on Steam.
Death to Spies: Moment of Truth
Missing the French version, which is included on Steam.
Dead Synchronicity: Tomorrow Comes Today
gog offers version 1.0.7 while Steam has version 1.0.12.
Amnesia: The Dark Descent
Basically unplayable on W10 if you have the FCU. This is fixed on Steam.
Shadow Warrior Classic Complete
Devolver added a considerable number of Alpha and Beta builds as a bonus to the Steam version for Shadow Warrior's 20th anniversary. None of these early builds are in the gog version.
Space Rogue
Missing patch 1.11 - it's more than a year out of date It's also only 9.99 on Steam yet 19.99 here. The price was permanently cut on Steam a while back.
Chroma Squad
Gog version is still missing the episode editor.
Ash of Gods: Redemption
Region locked in China, Hong Kong, Macao and Taiwan, where it is readily available on Steam.
DLC: Ash of Gods: Redemption Digital ...
Region locked in China, Hong Kong, Macao and Taiwan, where it is readily available on Steam.
99 Levels to Hell
The Steam version of the game is version 2.0. The GOG version is still version 1.0.
Double Kick Heroes
Region locked in China, where it is readily available on Steam.
Ancestors Legacy
Was up for pre-order on gog. Come release day, the gog version was put on indefinite hold, no new release date was given.
BLACKHOLE
Missing patch 1.9, originally released back in April 2017
DLC: BLACKHOLE: Complete Edition Upgrade
Complete Edition? Not quite: Missing patch 1.9, originally released back in April 2017
Dying Light: The Following – Enhanc...
It does not support the "docklets" codes for weapons and some of the new rewards offered as part of their new free DLC packages. It also does not support the companion app.
Surgeon Simulator
Free DLC "The Heavy" and "Trump" missing, because: "Due to how GOG works the build of the game that is available via Good Old Games is a different one to that which is available on Steam as it is DRM free. We'd need to create a whole new build."
88 Heroes
Has never been updated here ever since its release back in March 2017. As a result, it's missing both the free DLC RSG Champions and the paid DLC H8 Mode Activated.
XCOM: Enemy Unknown Complete Pack
Complete Pack? Not quite... No multiplayer in the gog build.
DLC: Finding Paradise Soundtrack
Does not feature the FLAC version of the soundtrack (which is listed as "coming soon" on the game page since December 2017). The Steam version of the soundtrack has a significantly higher file size.
Candle
No option to buy the soundtrack and artbook found on Steam
Elven Legacy Collection
No Russian version here
Finding Paradise
Missing Polish, Ukrainian, Spanish, Korean, Italian, Portuguese-Brazil, Czech, Turkish and Hungarian language. Missing the alternative version which is available on Steam.
Downwell
The current GOG update is "DRMFREE 20161119A", implying a November 2016 release, whereas the latest stable update on Steam was released in October 2017. Attempts to reach the developer on Steam and Twitter were unsuccessful.
No Man's Sky
Gog version is lacking the multiplayer component introduced in the July 2018 patch. Dev states it "will not be ready at launch; we expect it to be released later this year as full multiplayer parity remains in the pipeline."
Broken Sword: Director's Cut
The Broken Sword Classic version is missing French, Italian, German and Spanish version
Broken Sword 2: Remastered
The Broken Sword Classic version is missing French, Italian, German and Spanish version
ArcaniA
Gog misses many voice overs from the Steam version, even though they were allegedly provided with the necessary files.
Holy Potatoes! We’re in Space?!
Contains Redshell spyware. After being called on this, the dev released a patched version (v.1.1.3) of the game on Steam, but the GOG version is still unpatched (v.1.1.0--over a year old).
P·O·L·L·E·N
Missing a large update and a permanent price decrease from $25 to $10.
BATTLETECH
GOG users who pre-ordered require a 3rd-party account in order to use their custom Mech skins, the Steam version doesn't. Some gog buyers have reported crashes-to-desktop if they're not connected to the net when they start the game.
BATTLETECH - Digital Deluxe Edition
GOG users who pre-ordered require a 3rd-party account in order to use their custom Mech skins, the Steam version doesn't. Some gog buyers have reported crashes-to-desktop if they're not connected to the net when they start the game.
Kona
No VR support for the gog version.
The Vanishing of Ethan Carter
No VR support for the gog version.
Race the Sun
No VR support for the gog version. gog customers do get a registration key for Flippfly's website, where they can obtain a free key for the Steam version (which includes VR support).
Broforce
Missing update 2018.4. Numerous inquiries from gog customers have gone ignored, so it is currently unclear if/when this update will arrive on gog.
Overload
As of yet, there is no option for gog customers to buy the soundtrack.
Advent Rising
Received a patch on Steam that added a new launcher with additional graphical settings such as Widescreen support, Vsync, Trilinear Filtering and FOV. In addition, the soundtrack was released for free for Steam users. Both have yet to arrive here.