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.
Jippz, [N-G] [DFS] Glaceon, An Iconic Idiot, Suitableace4, Creeperman626, The Happiest Tree, A Combine Elite, $il3nt_P®otλg0n1st, LunarLancer, tf bot 3000, igorcurak, falseprophet141, ЯΣD_ΣX, alessandrog2004, JDKingOfWorlds, Isdras45, halllidvuu, Combine_Advisor1321, Combine_Advisor1321, aleksis.grinbergs, merkur7, iamkaixa, iamkaixa, slowdrag, Jantam11, fuck i suck, [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.Shark [relearning tf2], HiddenMonkey, Mrblue630, boblealeonard, THEWATCHER, knack03032547, CONRECON, Dragon Infused, TheTrueFutureG +3 others