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.
FishTahko, AdmiredPlague, itchy vagina, shefocrz, walaa_alhalabi, modadas12, Cat, YnX420Blazeit-Wordup_Fazeup, Business Fish, Madman300, Eggo, Stealelves, yamiakuma, Merlinmast, Shadow_Master953, ln3state, Chocolate Chip, MasterKush, maxart, aftab.shaikh360, MeshuXD, 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 +343 others