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.
wayne1, [email protected], Zingle_Weed, Dan0217, Superiorsound, DarkShadow126, Johson334, lonely19682005, George 08, Sentinel, The Unlucky Medic, The Unlucky Medic, LazerSpy
Submitted by garrysmod.org