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.
A Block of Aged Bre, (миша е2), Jet Fuse, Laugh'N'Point, OPEX, lee.gav, Exrize [AWP/SSG] SKINHUB.COM, Stealthy Stabber: SFM Posters, BlackRaven, Djcammo, Aero, Devils_monkey, Winter, ManigoHD, GodOfWarfare, The Real MVP, MY NAME A JEFF!!!!!, Sven, Colonel Calcium, cjackett, ✪Froidaxx [Neo], Stoner, Moonbat47, Nasom - Superiorservers.co, [AyK] Cyrax533, jordantroccia, dillonmarshall7, Animated DragonBite, Mr.Whoop, Zeratul, BynomenT, _SnowDrops | CivilGamers.com, Strike305000, Bubbles, Afireal, JagTeam Speed, Shadow, JagTeam Atomic, lchristophe54100, Makar, trimoore5 superiorservers.co, Onslaught, prometheus, Red-Velvet Cake, Red-Velvet Cake, Server-Gamer, Selganice, donaldé trump, Ayy, Slush +238 others