Garrys mod PROBLEM!! HELP!!!

[GmodeR]

I have Gmod 10 NON STEAM and im happy with it but the problem causes that i need to get addons. Garrysmod.org hasnt been able to download addons that simple, i have to register there to download addons. I signed up through steam but it doesnt let me verify it because it says i dont own Gmod on steam. So there no choice but to download gmod on steam .
Can you guys tell me how to install gmod on steam?
PLZ

 

[FishBulb]

You buy it. Seems you're in luck because it's on sale for half price.