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.
~♥Ketchøp♥~, leanizaguirre, od.danil2000, gold, GameRPlay09, re15revisited, ⠀⠀, pjgo, EgN| ArturoLevi, Sherry Stein ❤, Paton, [EclusionRp]_daftgmod02, DrakanJR, Vill Fite 4 Sandvich, Flamer, Tyler, Dawson, Dawson, Brightness, Zachariah, Durge, BattlePug, Mr. Cosmic Seagull, Слышь,Профессура?, stick of gum, Sovyet, Camaro, Warpath-the-drone swe, Warpath-the-drone swe, Warpath-the-drone swe, Johnny Five Aces, Echo Virus, Rude Ass the Red-Nosed Reindeer, gagecollins1138, zdishmant