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.
nottbnrblue, Neeexus, luigiVz14, I Approve, [Uranium] KodrexFR™, DrewDriver, 2241586939, erik.pimbor, galetinho2019, Kmdr Revan, Эльдендок, MIRA4N3, mymage3533, Bruhmomentz142, Mr.ProousMan(ITA), doritoperidot17, kawanssilva10, ItsTheNightBR, deathzone1332, rgeletey, Varus, unclebody, BMGaming|CH, ๖ۣۣۜZβø₦ẫ®ь, Devan ツ, spaghattosnoodles97, zaza123, terrorist man, YoungPsychoPlays, fellpopinha, leptitboss45, nishikiyama gaming, pmarcellojr, TheFuckingScout, periev.danil, Prostar, Yutter Nutter, jalalheidari83, joselp.jhsb, Debonair Refi, michael b jorden, [Evolve] Screella™, fakrul484, Jare.org, Snow, LINK2ChaoZ, Dr. Ermac Freeman, Spek, Sofiane-77, SharkFin +7 others