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If you have a Source engine game (like Half Life 2, Team Fortress 2, Portal) you can run this mod, which is basically a sandbox game where you can build contraptions, play custom game modes, and use content from all Source games you have installed.
 
I tried to get Garry's Mod off of Steam and they wouldn't let me buy it, saying that I didn't have any Source Engine games, even though I already own Portal and it is on their list of games you can use to run Garry's Mod with. I guess they really don't want my $5. Garry should complain to them about bugs like that.
 
I tried to get Garry's Mod off of Steam and they wouldn't let me buy it, saying that I didn't have any Source Engine games, even though I already own Portal and it is on their list of games you can use to run Garry's Mod with. I guess they really don't want my $5. Garry should complain to them about bugs like that.

Is it the free version of Portal? I'm pretty sure that doesn't include the SDK or whatever is needed, which is fair enough considering it was free.
 
I came here to post about Garry's Mod. Truly a wonderful gaming experience. And it didn't cost me anything today, since I bought it years ago. Good times!
 
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