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.
Bond22 Br, The Orchid Queen, giacomoromiti, sabs, kneesockscanty, mossesx, LoreLess_LoreBreaker, Salad, Team Fortress Türkiye (Adamyaaa), Empire04, scharlv, Electorate, alexshulski, Sturmfa1ke, Albert Sterling, Amvolret, 2karthiknair, EľKazel, tshashimal, darius.ajc, ViciousSARR, BlackSlayer, al.azizs, krychas77145, mirniy.sanka, ( ͡° ͜ʖ ͡°), Shooter, Bravo2YTpl, Varant CSBETGO.COM, jlcascante, papajohnspizza310, TheFiddle47, wswd009, KingKryptic, ryanthey, EryCola, 1149697558, DECOY panda1, karoldd9, It's called Hentai, ecekirlilar, webs, The4thSecret, jorgenieblas, Azog the Defiler, Vrbey, decadent_uk100, BloodierWrex, I am waiting for warhammer, caball0005 +54 others