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.
Krimskiy Grustilla, Mrcat2321, LUGO[Spec ops the line], Nuggey, Panic Manic, [FP]ICEMAN, oRavenix, BDOGGBOOM, Shadow, Shadow, Sgt. N Baker, JimTheCookie, ANDREW KILLER, ANDREW KILLER, StevenR380680, ikosmid