How is backing up an installer from GoG different in any way to backup a game folder in Steam?
Both can be copied to a different computer and used to run the game offline forever (unless of course the game has DRM, in which case both suffer from the same problem).
To be fair GoG selling point is that it doesn’t use any external software, it tries to emulate the old disk feel.
Personally I identify much more closely with GoG philosophy, i.e. mostly no DRM, manage the games on my own, etc. However I use Linux, and Steam has been investing into it so I’ll keep giving them my money (the input management is indeed great, but not enough on its own for me).