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.
J123oseph, TheDazzGame, qp, ckrist.almeida, Confused Foreskin, JINX, *****, NoWay, Mactavish, -|Sparky|-, Bull, CoreyHD, Sn0nik, ImGoodIKnow, Luke, TristanP13, Li3ToM3, woolwil, [FR] Matspyder, haikin, Dawson, [MyDarkRP] Nero, [ГУЛЯ] RedLaser, Architekt_LP, Keshwoof, Paytron5000, [RG] Fatulatti, LegendofRobbo, Engineer - STNetwork, kurtis_13_1994, [DR] Wally, MS Gameplays, No! No! No!, No! No! No!, Starvox, Mescaline, , Nyaaa, ゼーン(Almighty)