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?

frollard8 years ago
You need to link your orange box to your steam account - its permanent, and links your cd key to your steam account. Then, when you get garrys mod from steam, it'll recognize it.
west49rules8 years ago
To run garrys mod u need another valve game downloaded on the computer. What I would do is return the game and buy it off Steam with another valve game packaged with it