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.
Fruit Loop Dingus, Fallout_Fan, [Merg'z] F4N4TiQu3-60, ЛЕха, ZinoviL, K, John Mason, Taxin2012, Karma, Mrbox15 , The son of FuckBoy, BlingFantasy, StickmanJohn, codybrules, Kirussell, Nidolai, SmoKe, Mister, Nick Pyro, [3FX] Subject_Alpha