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.
killeurpro06, lDunnol, Nobody, andresdeleonpaniagua1997, Oriyame, Lauren Matias :3, xalokpasa, yanis, ya boi io, joltive, kmanblow, davidcaceres104, takahashikuroda, giggity205, jnoski92, Manulink, federico.laonda, sebasjfk4, RReiX, Brantendo, tygerfrogcity, Locket99, NightGlow, Sonic Alchemy