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.
[Ding], mossesx, joemari, Bosstn21TH, Texture/Fence/Fence_01, smwhitney54, Alien, nietonahuel211, Tange, Bravo2YTpl, dljackson2987, Vanoss74, (r4mbrix)Targa, j.dilla, Amir, Roman (NEED A MAPPER!), MMRHunter, Raptor Jesus, Helixframe, Hideo Kojima, Hideo Kojima, Sir Pickles, LBW995, ❖DarknessRingo❖, The Mystic-Man, Landy, Hayal, Hayal, ガノンドロフ, Milk West