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)
Basically just go the the folder and run bin\x64\Cyberpunk2077.exe
The "Launch Cyberpunk" shortcut in the folder starts Galaxy and then runs the game from there.
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.