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.
LiquidSpoon, Lolgosi, Lala San, HighwayKiller, ✪ andyItz, Lewis, Нормальный мужик, combatd3, Chester Copperpot, CommanderKnee, Axyd, Adma, Blimey, LGDaniel, Lamisen, Johnny's Kush, y}|{a(c), cappuzitos, Warthog, buildingapathy, reconditewarmowl71, loopyvoodoomagic, smeegle, Ms Draca, 룬빛싸랑, sokkahabdelkader, sokkahabdelkader, Maxi Senpai, walid93400, gabrielmanicraft945, Fedolgin, Fedolgin, 2dank4u, █▬█ █ ▀█▀, പോസാർടെക് സ, gabybarrocos, imgxamtfx, Unusual, mezzobuzzcontact, PhantomSFM, EnzoLeLama, Mongo, Mr. DusMees, The Goombassador, burnellbraydon2001, Kenvenski, U G A N D A N K N U C K L E S, love.stranded.deep, [OptimumRp] TomLesca, aaaeppoo195 +38 others
