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.
ScruffyMemes, MarioBros422, [email protected], szabolcska2007, R!OT ツ, (Ω)SPADt47, aleksansander, WaGe, Kẩm, PARTICLE UPBEAT, MaximusGG, lukewheable7, DeadlyIn720p, unbidgam1, Mountain Dew, jameswreilly1999, WEREBUD, Comics, MadcatMK6, ShadoW, Dat Boi csgospeed.com, arsenij6011, >>~~Foxa~~<<, ridecoastersallday, Azy Dreemurr, david.conev, haydenhargedon, Mr Sexy Sac Basket, FluttersIstDerBeste, Grozger Zasrogtiveslin, phenss, Magical4S, Pan, kevinfromgamingapex, Mega Man, MAsterpieMAsterPie, Guffyfish, The Man of Power, TolgiCraft[CZ], BlockBuilder57, Mio Akiyama, ElReyDeLaMilanesa, nst, smb123w64gb, [TG] MegaMinecrash, Quickscopulan, (Spooky) Troll, VitoVice!, Fidalgo, HamZang로리타분 +20 others