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.