Edit: http://www.minerwars.com/SteamKeys.aspx Apparently its one of those games where they have a separate login thing you have to create. Anyway once you do, you can get the actual steam key there, after entering in the key from GMG.
Just picked up Batman Arkham Asylum for $3.74 with that code.
“That's it. You people have stood in my way long enough. I'm going to clown college!”