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.
Krooliger, ⒽΞ⋀DR00Ⓜ | CSGetto.com, ASeriousGuy, Hunter, ZeroKite, ZeroKite, RaneZero, Jurisdiction, 88Deimos88, Valhalla, Bestgamer3, Alfons-Schuhbek, Mykootoh Nyoogi, Dopey, Dopey, Chippy.exe, Eshap, (2142)General Reaper[FuG], Jensen five, Crazy ol' Cheeseburgah, spongebob, 3dgy_Santa_Boi, Puggilicious, Eleven 24/7, Bob, golden freddy, Sokkah Abdelkader, Sokkah Abdelkader, Storm_Ocelot