Just log in here with your steam account and click beta to grab a key. http://account.garrysmod.com/

http://garrysmod.com/post/22706710505/20-000-beta-keys

Wiki about the beta: http://wiki.garrysmod.com/page/Beta

About 15k keys left as of writing.

Don't have Gmod so no beta for me.

@Danteveli: You're in luck, its still for sale on Steam. http://store.steampowered.com/app/4000/

Also, i am not sure how it works if you don't have the game but it's is a good guess to say that it wont work.

I thought he'd stopped working on Garry's Mod, been a while since I've played it to be honest. How is this actually working then? 

Thanks for making this thread! I've got my beta key and I'll be messing around with this later.

Bike gun.