Buying a game on Steam gets you the same perpetual license for your copy of the game as it does on GoG (the same as any software). The difference is Steam’s DRM (requiring the Steam Client to run the game). AFAIK Steam have said in the past that they have a plan to remove it if Steam shuts down.

Here’s a video that lays it out in detail as to what game ownership means amongst other things www.youtube.com/watch?v=tUAX0gnZ3Nw.

The big question about all this stuff is how far companies (both game and non-game software) can push their “you don’t own your software” agenda before facing a significant legal challenge and what the outcome of that legal challenge will be.