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.
QTG坑, filinho2308, kutikut4, WA | Herdapter, Lava0715, jose, noonoip, greensign7, asjad612, bmboy122, yah1, 팕민, Handsome Turtle, Gosht, rhys.p1994, chlrud951, HunterSNS, rohitktc, azamani0326, azamani0326, terrariaking22, mistercreep64, A bird, abolkkj, an.krawczyk, julesaug, alex, =PP= 419, rifky12_naufal, Mr_DarkBlue, ales.tomse, damontbridge13, TheCocasio, Fondateur | Kriax