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.
Conner, SkyyGame, Rayek, Rayek, Jailbait , |`m`☭~Ƥ ØØM~๏☭๛[TH], AT_LUS, PoulSn34KeRz, Twitch.TV/ARBALB, Script, CeramicKnife, Cal Lightman, www.squadlife.fr BadySmith, danilstarbird, Adamizer994, Karma, TNGevilcow, brodiecantskate, Bender, Twizorox, Skull, Randomly Random, OzxyBox, WhiteSasch, You touch my tralala, [FG] , [FG] , [MG:TM] Spoof, BillyPiggy, BillyPiggy, Simple BT, [TFT] Adrien Mojiitooo, Shark, The Unknown Particle ≤NT-A≥, [AGS]BrainyPlayz, [SCG] Aidan, [SCG] Aidan, [RG] Zeo, David231 (AW), ✪ Snowy, Wolfy_, Wolfy_, Moltag12, Crimson L, BagOfMarbles, Kat, AgentS2, [PP]hamsternerd, FuziOn, FuziOn +225 others