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.
The_Amputators[blw.tf], Lawliet123, lxʇsʎɹƆ ❤ ᴉ ɹ ∀, pitoonjiti4, STARK GAMES51, dEffy, shveshaguri, JayPlays2222, yakoub.djerroud, jeen_chayaphol, DeadBombix, MJM_Bld, diegoatari8, JesusMorales175, stas1211200656, hellcase.com | csgolive.com, yaroslavhrnayk, fplay5230, mossesx, SirBlond, rogerportnoy, lee.gav, piotrek.kucharski83, vasa.rus, joshmay143, NANOBYTE, Jay_Zcroft, zom-_-bies, Elemental_Skillz, Lord Demon, LoveMask, crazyhacked1234, Crystal Pepsi, Zen-kun (Neburo), dmaksar, Bilal3030, Bilal3030, Lucerno, samyon196, talhaqadeer82, egeberk1449, mto32msr, you2323stupid, CRYBOY, thailandtin2548, SpringBelarus, Painis _Cupcake, younescombat, Kokutso, s.sasha204 +280 others
