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.
saevan07, asierdlt, @gamler, Suprem Cmder Rê, Suprem Cmder Rê, Robingullstrand380, koni460, дракон, flocke 1123, Forley, Leddy231, [Y.Y.G] Brucektrain DEAF, [Y.Y.G] Brucektrain DEAF, TrYsX, andrewdbuilder