So garry decided to finally activate his Steamworks package (or it was just given to him, I'm not stalking him, I don't know) and one of the perks is an official game group!
Here are the reasons you should join it:

-Official
-Generally better than this one
-Maintained by garry himself

So yeah, join the new group at http://steamproxy.net/games/garrysmod

Garry's trying to test the new beta of Garry's Mod, but can't do so in Steam. Everyone can join in, as long as you're registered in the Facepunch Forums.

You'll also need a SVN client, such as TortoiseSVN. Read the thread for more information.

http://forums.facepunchstudios.com/showthread.php?t=441466

Have fun and remember to report the occasional bug.