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.
johndoesthrowaway1964, Missing Texture, AXSTHXTIC, csgofast.com™TeamRLX.G2A.Ке, 2241586939, TooSquish, PuniSh3r_LP, slavyanin, Nekyto, KilluaElRusso986, ุR, ka82deadpour, FazeAlanMskull, doom3dude, delerdroesh9, mario.meme.exe, mario.meme.exe, Trailblazer, TheReaperGame, jonq01, MIRA4N3, Pepe, -=Vethqype |CMBFan|=-, -=Vethqype |CMBFan|=-, mossesx, Esquire, ( ͡° ͜ʖ ͡°）, TryHardGamer115, Niv3m0re, Dragxter, alashi.mosterskull.monsterskull, archicastor1, Russell Lightbourne, Noble, hcolby0314, ToXiC JaKe, spaghattosnoodles97, [American Union state], [American Union state], [American Union state], tron1011021, LogyGames, Mr Soarin, Mr Soarin, Mr Soarin, Mr Soarin, Mr Soarin, Mr Soarin, Mr Soarin, Mr Soarin +56 others
