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.
Me, Bender, YOUPLABOUM, Jakew234, Gangstergamer7|CSGOPoints.com, InFernoxs, btwarrior, Bob, Бухачу!, RageGamerProductions, adenildacsme, [FLRP] Volrod, Bubbet!, Shadow154x, Jouji Washington, Becks [ARG], ●라이스™● [IRON], Remis, 제라드, gabrielviniciusalencar, chantry.conkle, djermoune1666, Jimothy69, Rellikson, Belamir, 猫精灵, or9311, ~~, CaptainSawbones, Coincidental, cryingenderman, [CG]‹<Tru>›Harley, edgarelcool, goodmage, minhyung0505, Church, rosemieko6, kon_talos, ph33x, Moneyy, KEEM, ☭BlackShark☭, ☭BlackShark☭, oliver38, [VXGGO]VORTEX, [FL] Avgar, [AussieLightRP] Unwoven Sleeve, The Joy Of Pugs, The Joy Of Pugs, The Joy Of Pugs, TroshKitty +11 others