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.
obibest2016, coocos122, dawid__2006, Repazbwin, Tyrex-senpai, 145 East Bindleton, Baid, francisco_lopez_1994, mrjohnaustin1197, ☜═㋡♛THE JOKER ㋡═☞(, francescoganzirro, Macka, major kopyto, Battle Scarred Anal Duck Cherry, ChinaMann, cornholioelmasriko, Frank Fiddlefun I Hyplex.co, shawN, mj01032003.mj, brian.mahotiere13, kam666il, Gibs141, jairebyers13, henryblackburn, Lharzor, illuminati, giuseppe_beehoe10, El Taquero 65, El Taquero 65, clwesty17