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.
Buller, thailandtin2548, titacool1234, EpicGod, ins4neik3, NicDXY, frosty, DDRP poenixskull, ☣ [PL]Cziterek[YT] ☣, Raiden, +CG+ Blue Potato, Zigi, Sassy the Sasquatch, Du Pimmelberger AMK, Zic101, McToady, Muhairbi, Coop!, SOLDIER-76.COM, ^6FaZe ^5Meme, archi, Console, EcoDoge, Combine, biohazardman, united, Kirbydude11, Heckelex, Behnzon, Stan, Connor™, ᵗʸᶫᵉʳ, LINK2ChaoZ, n0mI^', n0mI^', n0mI^', Bery28, EpicGod, Jaffa, TheBlueDiamondKid, Dylan, Dylan, Fr0stתf0rest, Imperial Knight, Matt, omy829, fexa, -=[UH]=- RyanJGray, Kahldin, Spideynn +30 others