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.
victorherrera, cortlandc, Aldana donscheff, ★Versus★, jhonmarstar, ChickenNuggs, jolenecarter41, filipesilvatabau, Paralhama, bkuloglu4141, , ap3xw0lf01, ChillAxe, Josep-Chan, fussyb23, Carlito1511, Carlito1511, atshasan, carluismc30, Genesis, Mamad Dixer, Mamad Dixer, NanuPlayer, NanuPlayer, JustRace125 #TF2SP Mannco.trade, JustRace125 #TF2SP Mannco.trade, nmla2009d, lena-konovalova91, renee.drozd, Spookachu, salchifelix, drake9306, sauce extreme, RoyitoElGamerYT, tuputamadre2006, :'), HappyYeet69, filip sergant 420, Gorkepork, taner08051977, Shimkokie, GingerbellDoe, GingerbellDoe, romantyczka64, MachinToutVert, fenix2, ardahanamatorligi, unclebody, Jabcha, steelbathsuicide +44 others