Garry's Mod 13 Beta Keys are available now for those who have played over 50 hours of it.

Garry's Mod 13 Beta Keys

Simply click the link above and sign in via Steam and then go to the Beta tab ( if it doesn't send you there already)

It will ask you if you "want one" and click it and BAM!
You have your Official Garry's Mod 13 Beta Key!

Enter it on Steam in the "activate steam product" tab on Steam and let it download!

Have Fun guys! :)