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.
Dennis Mcdickface, sergiomagana2013, LORD MAJSTER, аватар, SuperPoulbaGooper, jeanellebarrientos, a72672094, mossesx, kevineanr, kubacz csgolive.com, iunnoi滚, ☢Stalker39☢, Black Roxas XIII, Martian arachnid, Captain Charger, ytlol21, od.danil2000, Pomelo-Dilina, justinm, Blue Bonnie The Bunny, xXn00bslayerXx, Ashir_PL, Ashir_PL, alex04, Reactor (Albedo is best waifu), Scout, ScrappyBuzz, E7ajamy (home, doin nothing), (Faba's Student) Mmmph, gamerpushover93, Krobus, Krobus, Samy aissa el bey, Totomaster, I am waiting for warhammer, Sage, Andrew Ryan™, BFG9000, captsnow, Wolfman19682005, HZMT-Jahalaa, soviet comrade, PeculiarReality