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.
S1mp|3^m8, Mr. DusMees, viliemm, Jorgedr, fartblaster, KIR, Rolexx_Gaming | FlatForce, [RG-RP] Henry, damiano2233, dunno6376, Коты Атауют, JAISLON, MedNadjib, Okylel, Mofi, Ｋａｙｘｙ, jyrenjyrene22, BlueSkyIsTheKey, 가☞ (⊙)(⊙) ☜슴, [RG-RP] Paladus, ByPoLaT [TR]