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.
chrysler.email, AleXXX_007, Malurhan, pyro(CMBWRAITH), TOO-BEE!!, ValicOnze, [L²:RP] Bambo, 7atert0t, [L²:RP] Lord Octagon, DeadlyDark, greggameplayer, [Soul] Black Star, Eskil, [MWG] Commander™, Salvus, Vox Maro, ☤Asuna☤