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.
Lenny, gaben newell, Red Rocket, Edward, Firestorm, GORYGIRL1996, Gem, [email protected], [email protected], AdvisoryCookie, AdvisoryCookie, 178086057, zak.max.ray.stormclouds, TortaNaKaprovy, Scuzzelbut, Sandslash, L4D2Fan13 (Birthday in 5 Days), L4D2Fan13 (Birthday in 5 Days), Warpath-the-drone swe, Warpath-the-drone swe, WARBOSS GORGUTZ, Venom Snake, KotenokGAV, draconica, draconica