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.
LOSvenomwolf, Zaak36, liamsl.val, azerty199933, ☣Theros, tokenondro420, Sulfury, 89518091401, gigamgbo, aso0omi56, fedor, Ronan, I'm Myself, Shervin Naseri, markoniks20, Norwin, vladim mr Cases4Real.com, Doktor 12, Acecool, TDon123, Ndo360, Mr.Pompadour, Owain, Zeo Mause, CaLeb, Sotla2