So here's the deal: When the free Portal promotion was going on, I got it. It was my first Steam game. Now I want to purhase Garry's Mod, but Steam claims I don't have a compatible game. Upon reading, I found that the promo copies or Portal don't work with Gmod, because it doesn't include the Source SDK or something. I would just buy HL2 or something, but I don't really want to pay for it just so I can use Gmod, plus my parents won't let me get it anyway >_>. Is there any way I can get it working? Any help appreciated, thanks in advance.