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.
gtaVshit, GummiEnte, almnac, Ikarosx3, VaC ?, Ace of Spades, mindy, Aruś (Andar), Mati9goku #RAK, Cajun_Bean, [TS] TSWR, Juuzou Suzuya, Cedric, FrancisTehSeal, Leeroy, MikeOnTheMic, Czekoladowy™, Den, Red Apple, [AG]Marco15453:), [AG]Marco15453:), [MTS]milo2015, paul, SirKryiz, Moderate Hurricane Weather, Boogy, A family dude, hyPeK, Nipslicer, [AG] Archdemon, ✪ BaXeD, Panda, Omnivion, JSingDreams, Conner, Conner, Sir Ender, Theshinx32, COMBOY23, Harkness on TV, ✪ ConnorMeister, XStealthshooterX, Cal Lightman, serenity, Andx lp, Cyborg, floral, Biedrongo xD, Wadanohara, Nathan +1140 others