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.
Bacon Toast Guy, re15revisited, Detective MAGNUM, M۞A۞G۞I۞C۞A۞L, Captain Snyder, Utong ng Yakuza, Zeeko15, G-MON, Russia Greatest Love Machine, Biahwtrooper, FarhanHQ™, Magmacow, [TRF] Commander Potato, Nom_carver, Yukon, [BB2] Faith, Gnome of War, TheBananaThief, Syntax