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.
Kung_Fu, Ricochet Wolf | trade.tf, sagar, muduprelep, hcolby0314, unitra086, phong.007, rajnish2002mps, uscygg, yuutokun12016, Luge May Kamote, artem.bea, MasterBungler, yuuto, yuuto, yuuto, demonsoul, andy.chalupka, guesplane37, Azrtheal, Polymorphic Turtle, ★ Nick <3 ★, H2O Scorpion, sascha2912j, resistor58, bartosz16a3, bertisis, coNtagi☣N, Herr_Apfelbusch, farra222, chuck24, xXILoominati,420BlaizItXx, Wolfgang615λ, Alone,Black Hole