These two things are not the same.

With GOG you're buying the license to freely download a reusable and unlocked installer - with steam your game state is managed by steam in a generally intricate manner that relies on steam being running to run the game. Some games do install stand alone executables but it is vanishingly rare to find a game with an installer included in the game files.

In theory with steam games if there are no other dependencies (like dlls or registry entries) you can copy the game directory to a new computer wholesale but often times even that won't be worth much if the steam executable isn't running.

Please note, a lot of this knowledge came from me tinkering with the overlay a few years back, it's possible steam changed things to make the overlay and client dll more optional but those are usually hard dependencies baked into the game executable files.