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.
luciofwolski, EverestJAJA, virtualfrog1000, ArchAngelLloyd, CosyCat, CapitalAtheist99, fultonsheldon, nothing, inukaulitz, ~p~4D-03 | A. Tenshi, e1bb0r, deusdeus78, cesarguerra411, 0x00FF00lolwut, StormTetris, bbkblake, brumbo, Fran, adri226, makiman8283, huckwallas10vanvliet15, Austin5098, wwwklement52, Ori, cancer, Absinthe Angel, StarPK 117, Musty69420, [RS] Repede, SomeBruh, [email protected], nasubigta, moisesortizmontoya2016, Dogamy, slavamajers, xhoqueen, masterchef, Fat Scoot, chenonjack, mireyaruva72, DanSuplexCity, AvocadoWasFound, EliteGoneWild, easedaed, Tiniest_Galaxy, jakubl0430, nicollasvibe, Uoufi, samurai-ric, PSYCHOFAGGUTS +1553 others