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.
rubensolorio94, davetrotter24, G-(er)man, Snow, KiryuBT95, Glorious, WolverWind16, cody93, artificialstupidity00, yabout71, Shiron, Himsky, Himsky, Himsky, Himsky, aoyagi9196, Outland Predator, ProZach
Submitted by garrysmod.org