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.
asdfghmailcom, Lui Calibre SuperiorServers.co, [FT] Sh4rkKiller, Erlik, [SilverSword], Obli, foriot, nitram1509, Green arrow, adasgadfg, [SNS] Xwatch1009, Stivenson 00999009..., Traffic Cop, YT-BlaZeRsHD, MyNameIsRed, Mangle .exe TF2, HoussEffect, pindjouf45, DENUGAN, pookeybear2004, Alarm, MK | GalactiK cases2x.com, Commander Shepard [SR-2], Evodeus, jakeboyharris, FiLeXx3, Super Micronde, rouxsexy, , BilliHorizon, moviemaker443, Krossfire teamspeak.darkgames.fr, BS - Rainbow Spectrum, General. Aesthetic, , WabelHD, ̣^ίήνάĻίđ, firefox450, Keytto, KraulT, KraulT, KraulT, CaLeb, Neolys, Anorexic Penguin, [GGS] 92ℭąɾɱɳąɗ, Travis Bickle, cygiel