Shame that Portal of Praevus isn't included.
There is 'The Ultimate patch for the Steam version of Hexen 2' that includes 'Portal of Praevus'. I bet it works just fine with GOG version:
https://hexenworld.org/downloads/archive/KUBA18i/
Features:
- Compatible 100% with modern Windows, both 32-bit and 64-bit!
- You have an easy to use interface to access every Hexen 2, Hexen World, mods and dedicated servers from a single place! No need to go into folders or play with command line parameters!
- Uses Hammer of Thyrion, probably the best source port of Hexen 2 around. Includes countless bug fixes(no more irritating crashes!), ability to change resolution within the game(no playing with parameters needed!), ability to scale HUD and text for higher resolutions(no more pain in trying to read what's on the screen!), folder based MP3/OGG music support(no CD needed for music), and it is fully compatible with vanilla saves and mods, so it's quite purist friendly.
- Includes a fancy new title screen! There's one for the base game and one for the expansion! If you dislike it though, you can just delete "Hexen 2/data1/Autoexec.cfg" and "Hexen 2/portals/autoexec.cfg". It's only a demo that plays when you boot the game, so it's not really purist unfriendly or anything.
- No more stupid silence! Hexen 2 has MIDI music, but it's really not as good and atmospheric as CD music, so after hearing CD songs you will not want to go back to using MIDI. This patch contains them.
- Includes the missing expansion pack, that's not included in the Steam version!
- Includes every map or modification for Hexen 2/HexenWorld ever made(well, at least every one I managed to find)! Mods can be easily accessed from my custom menu, no command line parameters needed!
- Bonus videos and other files included! The "Extras" folder contains the missing intro to this game, make sure to watch it sometime!
- Expanded "help" files! Now includes list of commands, Official FAQ and Mission Pack help!
- Setting up multiplayer games is much easier now!