Assuming most of you either know or don't know yet Garrysmod is having a beta for its next release. Consequentially you can get into this release by signing in to the following link through your steam at
Click on the top tab and hit the beta button. It may take a couple times to make these tabs appear.
This should work assuming you have the original garrysmod on your steam account and if not it may still work.
Once you click the beta tab click on the button want a beta key etc.
Go to your steam library hit the games tab, hit game activation, continue continue, and enter the code.
*This was written as I didn't seen any article about it on D-Toid and also as a general heads up, apologies if it's duplicate news however*
And for those of you who got bored the New Beta Added support for Portal 2 and Left 4 Dead 2 in Garrysmod. (Although I tried Portal 2 and it wasn't working but that's probably since I haven't played coop yet.)