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.
Nekyto, keremtr144, SukaBlyatHARDBASS, SukaBlyatHARDBASS, Demoman, Demo CJ:Here we go again, Demo CJ:Here we go again, BadGreen, D4rkFlyer, Bmestep14, Ashir_PL, RazetheBeast, nurlybek, BlackBurn915, [HUN] Shadov001, The Hoffmann, THE DEMIGOD, PsyKoTMK, FD Daylight, Jecop Lonsan, Jecop Lonsan, ❄☣Lord TL☣❄, Fiefio, Jurisdiction, poketwinsblackandwhite, Tumor-Inducing Grapes, Commander Toros (KPAvids), XtremeTerminator4, XOFDireWolf, XOFDireWolf, Scho-manno!™