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.
gasmihamou626, azzaheer253, mincenkonikolaj31, hooba, pardinimaxi658, f3raslol2, bankkrittipan, snylard, xgalvez928, blacksongoku05, mataron a kenny, ffanasse1, JackExe, carobri0480, Juventus, 2ncline, Fortune, БухойМажор, yurifurrj, llanepanela, freiteznunezsantiago, boadaalberth, Akita, alvarogaitan47, chihein2011, snowy.juegosnose, roaming, pr3696224, raccoonkiller114, Kapria Special Operations, xXxDylanPro726xXx, lord_demond, Mini255Coler, FatWolf64, JuanDMHL25, blackfireh.12, Luisagustin2946, dyzanb5, berkemc, pointblankbeybala, DANGER, diereci2019, Polloxd, artin.valamehr07, nicklasferro9, Jonatas1554, Sketchhog (linux *******), Ayman_7210, Che, tomaspereiratp88 +1625 others