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.
Aleksey3236555446, ultraplush1987, Totenkopf, Hitler is da best :3, jonathan.kamysek, Lemon Ice, Pikachu, Fox, FOXCMG :3, __Fluffies, ciper89, Cybernetic Alex, XiloFox, frefox2033, FoxHound1998, FoxHound1998, cododd7, PunkPredator, GameAct3, bartosz16a3, jcmh1995, Pfc. Amarikano [1-509th], Murder Is Pink, Riha.Jarous, Riha.Jarous, WI A ELECTRIK)))))), creeptasticman, S80LAR, DoctorMelon TheChocolateWolf, sparrowstar, Blue the Fox, Dander Woodwere, Rdovydas007, Badhorn Stadnent, [Wolfscast]lightwolf25, bunny, Micheal, Ruby_Drakojan