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.
DER REDHEAD ELOFICKTS, GreenImpact, Josh, Rambo is gay, Axam, djjhfgsjhy, Panda❤, Pieanator9, Leonkennedy20001, Mr. Bud Muffin, Mustafa, Lorde Mar, jakefrostyyt, Pedross, ironsmax1998, jc, FiecyLick, Shark, Coolin, cheeki, kenshie345, CHEESE BONES, abd.omari99, marciokalillopes, Nova, SacredAI, Seasonal Sky, DarkPlays, ᗩℕᗫℛ£ϒ_ℳØᖇĐ₳, Hidõ, Ghaw, jonah_molina, EyalTheAfk, jf, Redmond Viscaldi, Nate, Laeye, Edgy White Dabs, benjaminjacobson115, J4cK-2-8-1, ToomyBoy | Pvpro.com, plonty74, Kubík05, Vamist, Hyzian, Hyzian, xAsher, xAsher, [LP] InfernalPanda, < blank > +867 others