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.
elQUESO, Ghezzu, jafet.nimrod, joxman_159, cigulinin, Hunter, The Seer, What's Really Good?! nyuggah, Nightmare, juandavidchaparroarroyo, (Loner) ShadowOswald, luquitas rys, daniel.kiryjczuk, ToxicGamer, azamani0326, Xx_TRICK_STABBER_xX, Red Hex