"A Source engine game is required to run mods"

Getting this error when I try to run Gmod. Just got it gifted from a friend. I have Portal, which if I am correct is a game that uses the Source engine, so why will it not allow me to run gmod? Any suggestions? Help would be very appreciated.

It needs a source game that needs to be paid for. All paid versions come with the SDK that Gmod runs on.

The free version of Portal doesn’t have the files needed for it to be recognized by GMod. You are going to have to buy one of the games GMod requires.

So the free version released by steam for the Mac promotion will not work? Even though it is the full game?

That’s weird, I mounted Portal into gmod while it was free…

Anyway, sounds like Valve should fix this problem.

Mounting uses game files, whilst running on it requires sdk.
A lot of people have been doing this.

Things like HL2 and CSS. If you’ve seen screenshots, GMod uses a lot of HL2 Graphics/Models.

I’ll just presume that you mean textures.

http://www.facepunch.com/showthread.php?p=21967867#post21967867

Sure. I think anyone with enough sense to know synonyms would know that.