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.
TrainingMike, jebač godine, Amy Rose, Rincewind, artem.zaharov.2000, Mr.Freeman, I heart Nishiki, armymen12002003, Redd Jankowski, falseprophet141, falseprophet141, DECOY panda1, DECOY panda1, Harold Keegan, mamma.knullan123, Pantherlily, B1. BattleDroid, Reemiel, Armachem, Armachem, Point Man, Sharkydorky1, The Point Man, Jones Bones, Lil Ol'Gnome, Lil Ol'Gnome, KRUG™, Now Gaz "Battered Face" Helmet