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.
Shade Hooves, UNSC Dawnforge 2142, storm, GamerMan1516, That By fley, John Dakor of New Eden, [R3Z] ThatGamingWolf =^-^=, [R3Z] ThatGamingWolf =^-^=, AO | Dovahkiin, Hot Freeman, DonkeysWTF, F_Synchro, falseprophet141, falseprophet141, Libearium, coleman, NxTGeN, Ultrastar 1, Piro Winfield, PANDAA, PANDAA, straightontarget, Frisky Piranha (Magus Janus), Suddenshotty, снайп1910, OmegaZero, NightExcessive, Oskar, Reck, Rexy's Number 1 Fanboy, e r n i e b o y, Commie Bearlord, Hisha, T3ss3R