GMod Buying Error

[fierscream]

I try buying the game Garry's Mod, but this error pops up:

Your transaction failed because you are trying to buy a game that requires ownership of another game you do not currently own. Please correct the error and try again.

I don't know why. I looked at the games needed and I have one of the games in the list.

Half-Life 2: Deathmatch

Do I need a different one or something?

BIG EDIT! I should add, I got the game for free, could that be the reason?

[Ad Bot]

[Harm94]

If you have plain old Half-life 2 then make sure its installed, if it isn't install it. If it is installed contact tech support. Don't have it? Get it. Also you will want to have some other source games.

[Colten]

Its not valid unless you purchased HL2DM
If you were gifted it or given it for free it wont work