Quake 1 and 2 are complete here on GOG. Quake 1 includes the DOS version, the windows port Winquake and the (unrecommendable) OpenGL port GLQuake. It also comes with both addons (Scourge of Armagon and Dissolution of Eternity) and, unlike the Steam version, also includes the soundtrack. Quake 2 also includes both expansion packs, The Reckoning and Ground Zero.

For both Quake 1 and 2 it can be problematic getting the soundtracks to play, but that's not GOG's fault and there should always be a way to get it working :)


As for multiplayer, I'm not sure, but I've heard you may come across a player from time to time, but I doubt there is an active scene on random servers. Unless you plan on playing with friends I wouldn't buy the games just for multiplayer.