I played through Gateway and Treasures, so a few questions and writeups:
Gateway is a really underrated game. It's a great starting point if you don't want to deal with Pool of Radiance's lack of features (though Pool is such a great game).
You can actually play the same party from Pool of Radiance > Gateway > Curse > Treasures > Secret > Pools of Darkness for an epic 6 game romp.
Some item corruption can occur, but it's not a big deal.
How do you accomplish Treasures -> Secret? The character file formats are somewhat different across the games, and SSB does not recognize TSF character files.
https://goldbox.fandom.com/wiki/Gold_Box_Forgotten_Realms_GOG_Transfer_Guide seems to suggests you can only export to POD from Treasures (without some conversion).
Anyway, I once again tried Gateway to the Savage Frontier, and while I did try a run some months ago, this time I intended to play further, and I beat the game. It was definitely not a bad experience at all. Here are some more or less random remarks and thoughts I have about this game:
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-The economy is not as broken as in most of the other goldbox games. Only towards the end did the economy feel moving downturn, but it was still not too bad (compared to the other games...)
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-After some enemies (more towards the end of the game) diseased a char in my party, I cast a "Cure Disease" spell on him and he seemed ok. But the character had lost some STR while being diseased, and that attribute wouldn´t return to his normal value, untiil I cast "Enlarge" on him. After the initial effect of the Enlarge spell had worn off, that character would regain his normal STR value.
On economy: I agree it was an interesting situation when you actually didn't have enough gold pieces to train your characters at start. I took an "easy way out" and resolved this by going outdoors to the hills nearby and killed a dozen or two parties of hill giants. Very easy to get money and the early levelups (felt almost like cheating because the game was so easy afterwards).
On disease: the mummy attack, when it weakened the character (IIRC it didn't always happen; you could get diseased without it as well) resulting in STR permaloss(?) was annoying and I couldn't figure out how to solve it. (So I tried to avoid those mummies.) Not sure if this is a bug or feature and whether there is supposed to be an actual way to fix that instead of resorting to Enlarge (nice tip!).
A few notes on GSF/TSF run in contrast to Curse/SSB post earlier:
A small writeup on reflections of Curse and SSB (POD still pending, I don't expect much surprises on that front):
I ended up skipping Pool of Radiance, because it does not offer paladin or ranger as an option, and I didn't want to resort to hex-editing the characters to a different class in Curse.
So I started Curse with 1 paladin, 3 rangers, 1 mage and 1 cleric. Dual-classed one L10 ranger (after Zhentil Keep) to mage and did some grinding so the mage obtained L11. The rest I left to be dual classed in SSB
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Ran with Champion difficulty. I wanted to try something different and picked 2 paladins, 3 rangers and 1 cleric. GATEWAY was easy even without a mage (you just had to use bows to shoot down enemy spellcasters). Likely the computer AI scaled down most of the encounters for some reason (like in POR). The whole game resulted in about 200K xp. (NB. Cleric level cap was ridiculous, you got maxed out at 27K xp! Likely due to the developers not wanting to implement level 4 spells.)
Continuing in TREASURES, I ran the party maybe 1/4 way (additional 150K xp) and when the rangers reached level 10 at 325K xp (max level with 8+4 hit points), I dual classed one ranger to mage. The mage reached level 11 and unlocked ranger capabilities somewhere about Luskan so just in time to get the benefits before the endgame. I had the idea that if I were to continue to SSB, I would switch paladins to mages when they would reach level 13, the remaining two rangers to mages when they would reach level 15. I also had intended to switch the cleric to mage at some point (maybe at level 11 or 12, so it could in the end access "heal"); preferably I would have wanted to run it longer, but I could not dual-class it the same time as the paladins because then I would not have any healing capability in the party for a while. If this would have been sufficient wrt. clerical skills, the party would have been even stronger than the previous one because 5 characters would have warrior background and everyone would develop to a level 30+ mage in POD.
The first phase of TSF when I had no mage at all was not too difficult. Maybe the "fixed" encounters were somewhat scaled based on party strength (for example, in some cases the clue book said an encounter would have "sorceresses" in plural, while my encounter only had one). It may have been a coincidence, but almost immediately after dual-classing one ranger to mage, the enemy spell casters seemed grow in number. At times having just one mage required attention.
In TSF the most tricky enemies were certain black-robed mage and spy encounters and springing up from every direction. But a couple of uses of haste and regular buffs (and/or luck) dealt with this rather smoothly. The end battle was much easier than I expected based on some youtube vids and cluebook warnings; maybe I just got lucky with the initiatives, because I only lost one party member to a slay living spell, one of them was held during in the first battle, and only one was dragon breathed down in the second battle.