This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Mohatoons, please, Division | KS |, =WL= Lil' Magic | trade.tf, Radical Revy-chan, YEAH THE STUFF GETS IT, .ky, Jonni, gorshov04, Binary Wolf, Vioxtar, Alison Quinn, 4chon, KalaPihvi, Leafdroid, looper22, Math, 혜인, Fatmeatball, Fatmeatball, Smitty McShrooms, Smitty McShrooms, lolle, Lucian, Thendon.exe, Toroko has a gun, CheezMan, Hoff, Hoff, Teh Hatein, Mr.MustacheCat, Agentlulz, ｘｕｌｆ, ｘｕｌｆ, ｘｕｌｆ, Buu342, Tommy228, Tommy228