I just downloaded Quake here at GOG.com even though I own a copy of the game and its expansions on Steam. Years ago, I used some patches from Steam (I forgot the author of these patches, sorry) to enable widescreen resolutions, enable the music and other graphical enhancements. I uploaded these patches again in this site for convenience. Again, I'm not the owner of these patches - many thanks to whoever made these.

NOTE: Please delete/move the "opengl32.dll" file before installing as the games won't run if this file exists in your Quake folder.

For the original game: https://mega.nz/#!jkpmiYxK!bBdK3KZDOePOiTFL6xw3xD4O_Z4w6kgzZYNAQGvgwm8

For the first expansion pack (Scourge of Armagon): https://mega.nz/#!q9hkWLrK!GUuiW8dpwl9igxW2uNTX6cQjO4yQ9tTfjEZpjfra4Jw

For the second expansion pack (Dissolution of Eternity): https://mega.nz/#!mk5QBAIb!xA92K5uQAas5NOEiJIdT3jVesdoDCf-TkdiCcaql2Ns

Awesome, they work fine on the GOG version. But they seem to use an old version of Dark Places

Yep, it is the April 2010 build of DarkPlaces + the ripped soundtrack music files.

Be careful, even after after I manually updated my Steam version Darkplaces install to a more recent version of Darkplaces I encountered a progress stopping bug in Episode 2 Mission 3 of Mission Pack 1: Scourge of Armagon, and before updating it there was a progress stopping bug in the original game's hub world, though I think you can use a no clip cheat to get past that one. That was last year when I played, so I don't know if they fixed the add-on bugs since then.

In these cases you're most likely seeing bugs in DarkPlaces. There are many cool things about DP, but it sacrifices some compatibility with content outside of the main Quake campaign, especially in older versions of DP. It's just DarkPlaces and the ripped music. Once you have the music set up, download the latest version of the DarkPlaces engine and put it in your Quake directory. Ta-da you now have a more recent "Ultimate Quake Patch". :-P (Note: on that download page, you want the "DarkPlaces engine" NOT the "DarkPlaces mod". The mod changes some other things.)

If you're playing Mission Pack 2 you may also want to get this fix: http://quakeone.com/forums/quake-mod-releases/finished-works/6246-mp2-fix-darkplaces-plasma-launcher-not-firing-properly-updated.html (I don't know if it's still required with more current versions of DP.)

It's strange about the plasma launcher bug, I didn't see any problems while using it.

Just the heads up, the Scourge of Armagon music from this "Ultimate" Quake Patch is badly ripped. Each song does not have a beginning part as they are moved at the end of the another song.