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.
Purple_Penis, hyplex.co [UK]SYMMIJ♥, David Macintosh, Sweet Bro, ilten2002, stillnotmatthew, Flats The Nice Guy, Partially Autistic, Silver One I #PMOTF2, [YCTU]Dr.Kush, Peguinman12, DoC, ◯, O_Seagull, SuKaH :], ☆нєяσвяαн★, fulla, J., Chef, ReallyCZ, || Kawaii Killer || Nope ||, TheAntondi, |M:G| Grizzly™, Guest($), Deadpool, [ ], Jesumai, ReversoSpart4n, Corporal Cactus, Mitchelskater, [QTG] AveragePenguin |, ✮ ’REQUÂDRE | MAKİNA, Rayman, Owen, Ree, Bloodwake393, demptch, iiCookiez, xigbart, ClickX, Hectorator, Šcrä-[P]peŘ (Ban 7 Days -_-'), _67, Krush, Witchdash, Hectic, ┼|§♣|FeAr|♣§|┼, sniker, sharniej3, Fwuzzy +147 others