GOG is hardly a toy and is the platform I look to purchase tons of games on instead of Steam (which I really like) and definitely over Epic (which I've never even installed)
Only drm free steam games. The ones with the steam drm require steam client to be running to launch (steam itself can be in offline mode but it still needs to be running)
Games using things like steam input might also require steam to be running so there is some drm free games that might not run also. Some of those will if you move them outside the steam folder / rename Steam.exe. If you leave them in the steam folder the game will start steam for you if when you launch it.
