Look, with things like the Goldberg Emulator almost all games that use the Steam API can work without Steam as it provides you with a drop-in replacement to the steam api dll.

The main practical differences between Steam and GOG is are:

• You need to have certain technical skills to work around Steam’s (often very weak) locking. Not crazy high (basically how to navigate a filesystem), but some.
• In Steam you do NOT know at the time of the purchase if that will actually work or not (games heavily integrated with the Steam API still won’t work with the Emulator) or if the game has or not further DRM, so you CANNOT make an informed purchasing decision in terms of “will I still have access to these games in the future no matter what”.
• You know for certain that games in GOG have no DRM, theirs or from the publisher’s, because CONTRACTUALLY GOG forces the publishers to not have DRM in their games to sell via GOG.

Personally I buy tons of games from GOG and only a handful from Steam because I do value the certainty that if I have the hardware and OS for it (or an emulator), I can still have fun with those games 10 or 20 years in the future. Then again I’ve been gaming for almost 4 decades hence have enough experience with getting to a point were I miss a game that was fun but can’t run it anymore.

PS: Funny enough, my latest return to sailing the seven seas was because of an oldish game I have in Steam that wouldn’t run in Linux with Proton, probably because of the original DRM from the game itself. The pirated version runs just fine. I strongly suspect that if that game ever got sold in GOG it would also run just fine in Linux.