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.
Quakeguy., $il3nt_P®otλg0n1st, LunarLancer, igorcurak, falseprophet141, alessandrog2004, JDKingOfWorlds, Isdras45, halllidvuu, Combine_Advisor1321, Combine_Advisor1321, aleksis.grinbergs, merkur7, iamkaixa, iamkaixa, slowdrag, Jantam11, [DXG]Hunter, Shady Figure, carsley9, Jey, Blades, AleGamessita [Csgomassive.com], camry, FOR ADOPTION: (Console), Daniel, madi pas, pim15, Cremator, Naruto el Noel !!!, The Pyrobear, Vengineer, KingBo0?, HiddenMonkey, [PDF Sgt.] Mrblue630, boblealeonard, THEWATCHER, knack03032547, Dragon Infused, seanbut42, seanbut42, snow98, Dop3KinG