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.
gryZor4000, wiesbaden203er, wejrom, -1, seonbinchoe, Temqe, Jodie, Ethan, Meido, Scritch, SuperGamer23, Santi ARG RIFAS CS:GO, lkeymayoral, Super Jumpe, arcor777, clarkorino1, trolex180angel, sotirovits, Sskewer, echtmerlin, FunPlay, Mentrooo, 2564001131, iyrii356, Atomic Legend, Danny_Strong-DE, Parker, brandon, hankhank0202, Captain Kitten Fart, earthfase, J., NaegiGamezz, itsmakeshift14, Dhxhe, marvin.septim.98, migueter433, Valenchenko Kieve, MichaelGDad, cemil2_1992, Dragon Of Danger, pollo.ettore2, Backstab me, Leon, ExproMC, totallynotTim, Cardian, Faby, ButterMyWaffle, :\/ +87 others