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.
フアンマヌエル, SzexiBálint, mineeddy.754, BreakAwayMist, Tom, deak6174 @GamageR.com, stonesimme, TFGAlvin, ironhammer, [ßøß]Hospi, [FGC] BLACK SKORPION, Cmdr. MacMillan, oscar.courtney, FarAndLight, ☣pandaN2005☣ ͡° ͜ʖ ͡°, Ogait Das Neves, TheCobaltCrafter, prabasa09, •=ArtGamer=•, _Fox_Hive, lanierr05, ziyad3211, cooldawngamer, Tonny_Dragons, Rex5151, Gamer Snake, daddynene, imapirate, D3AD (Gary Sanders), QTG坑, Infinite, dremzi2002, phanshocraft00ara, ☃❆[SM]RazorNight❆☃, shulicXD, AceZ, hiorakreb17, MexiXxl, the_solider123, mrpigman72, B1ts -> Going Back [adding], The Deadly, MC_Warrior1, ☭vlocKer☭, Right, Eclipse, Mir023, SuperUser, DruDruGaming, Miami70X +46 others