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).
I don’t think there’s a way of checking how many games are like this, but I find that the majority of games I’ve tried doing that just work, and the ones that don’t are mostly bad programming (e.g. crashes trying to load the steam library).
That’s not entirely true, as a general rule I think GoG has a lot less DRM-ed games, but it’s not 100% DRM free like they sometimes claim https://www.gog.com/forum/general/drm_on_gog_list_of_singleplayer_games_with_drm/page1