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.
mossesx, bg_productions, RaCc0oN, RyonicFire, RyonicFire, Nooka Cola, Daniel_hero_5, austin_autsm, Theflax778, Samsouale, Redsparten321, GamingFlaming, SnoopyDodo, RandomColourzzz, TheElectroGhost, Luke Woodside, Yepco, Wolfy_, R4K0, Yermak, Marcos, Robert Bearson, João, kaelynj17, [HTG][FDRP]AzrahYT8, _Doctor_Gunner_, bubsy bobcat, FiberGaming EU, ZombieSniper230, KaiL, caerylin, Phosphene, SonySeva, aiden, SniKaM, Teddy, Jeff, [Ru]Dolf, Nexoh, [ANRK] Josh | ANRKRP, =PP= 419, SuperOllieWorld, Mr.Belou, fren, [TLGC]patrick8996, vin, KountKuntula