the requirements say the HL2DM does not have the source engine to run garrys mod, u gotta get another source game if it dont work
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half life 2 deathmatch is compatible with garry's mod alone but you can get a source engine just in case. cheapist i think is half life source
You can but it from steam for $10 or so. and since you already have HL2:team Deathmatch, you shouldn't need to buy a game with it.
I'm on a school computer right now so i cant give you a link, but if you google Steam its should be the first search result.