I bought Portal retail, but when I try to run Garrys Mod, it doesn't detect it as an official copy. What do I do?
I bought Portal at my local Target, because I had heard of Garrys Mod. That was before I knew about Steam, however. When I tried to run my (free) copy of Garrys Mod, it said that it (Portal) was not an official copy. What do I do?