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.
mossesx, <SSFD>Black-Beef(My Corse), falseprophet141, Spartacus, Zvonimir, peke_20_95, Cuboidjunk, Ivan, l.maire c, Fenix Legion, Warpath-the-drone swe, Warpath-the-drone swe, (2142)General Reaper[FuG], Reverend Jeße V92, The Insane Hunter