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.
Shadow_Master953, ln3state, Chocolate Chip, MasterKush, maxart, aftab.shaikh360, Dan0308, ɹoʞoqoouƃǝǝ, kalwani.rachit1, Crazysniper, CukiJam, skpacman, Vivi, Flexn, Trauma, Zeke the Plumber, Beetle_Lad, ThePunGod, 女 Ralu | ànémïy #J, Stuart Dubois, Mantelis, Epixauce, █████, tlsman, adrian.rotarasu, jhayw3, fendiking17, HiddenBlade, Ri_oT, MrTwistT, Aziz, LLaci3, Shelley`, Sotius, Ravenpaw354, PanImperialista, That mistake was Gold, Amarog, HαZҽ™ kickback.com, cwieri, PointAndFail, Xfos, Dash Alpha, glpinto, Varant CSBETGO.COM, admin, bdubyah, politimannen45, RGK, glenn_utley +328 others