The expansions don't have separate installers, so without some serious tinkering it's not possible, I'm afraid. It's impossible to buy the expansions separately from the main game however. So if you have expansions you have the main game here on GOG as well. Just install the game and you'll have access to the addons. Why complicate things? :)

Well i got it working

all i did was move both xatrix & Rogue folders to where i have my Quake 2 installed which i did from a CD which i still own

I then i created two Quake 2.exe shortcuts, then i went into the properties and on the Target line i added "+set game xatrix" or if you want the other Mission Pack put rogue instead of xatrix.

Glad that you made it working. :)

And what the point you move missionpacks from one quake folder to another? If you want audiocd music you dont need any specific versions, you know. You just need any audiocd in first letter cd-drive (even normal music discs is work).

GOG version is a broken completely (exactly mission packs). Properly installed v.3.20 is required (AFTER mission packs installed to quake2).

So i can play the mission packs without resorting to piracy Waaaay ahead of you there. The Quake 2 that i have installed is 3.20.

What piracy if you cant buy digitally addons without buying base game first. it still nonsense what you saying and doing. You move addons from quake2 folder where you already have quake2 to another quake2 folder. From version bundled with OGG music to version without music until you properly mount game cd image or insert real one cd. Better quality music? It still available for GOG - any AudioCD discs will recognized if you put it to cd-drive.

But for real discs you should rename real cd-drive to first letter. For mounting you can use first letter for mounting software.

And no, "version that i have installed" is not what im talk about. If you not new to digital-versions you should know that. GOG sell 3.20 as well, but it not properly 3.20 build. Addons too. Better extract-apply patch again (since it self-extracting archive), not matter CD or not. Patch should be applied after any official expansions.