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.
KEKW, daniel.chatillon0921, Tails19935, ZONT, re15revisited, Emir, alexander.morga, DeadDPpool, Kekoa808, Rowie, nongteam_seka, Bobe, Matt-Kun, Engine Company 37, LittleXen, Monkey D. Luffy, PotatoJoe, Skeleton , GOpro, Higherson | Roadkill.com