How to install HD Mod in Quake and Quake II GOG?

Do you mean this HD Mod:
http://www.moddb.com/mods/quake-hd-pack-guide

If so then there are instructions in the Zip on how to install it, they are for the original and for Steam, but it should just be a matter of replacing the path names for that with your GOG ones. I assume you browser downloaded the files, ran the main file, and installed Quake to somewhere on your C: yes? If so make sure its out of the windows area, e.g. put it in c:\games. Then (depending on the mod, and I am not familiar with this one) you either extract the zip and overwrite the files in your game install, or copy it to a specific mod directory, again check the instructions.

I may have a look at this when I get home.

Edit: Or maybe it is this HD mod:
http://www.moddb.com/mods/quake-hd-remastered

If so, according to the instructions you just drop the files into your install location.

HI guys,

Can anyone advise how to install quake2xp mode on a top of the GOG Quake 2 Quad damage. I followed these instructions, but it just shows a grey screen:

Installation:

Install quake2 from Steam, GOG or from original CD-ROM.
For cd version install 3.20 patch.
Unzip q2xp in to quake2 folder.
Install openAL drivers for your audio card.
Remove previous version of xpconfig.cfg and CACHEXP folder.
Play it.

I didn't find "xpconfig.cfg" in my Quake 2 folder as well as how to install just Quake 2 as it seems that most graphic modes do not work for the extensions anyway.

Many thanks