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.
Asus™, KRiSTiAN, TOPVL4D, mossesx, laDolan, Boomkiemon, Travis nowhere, a26b7e9a0ff67b35fa4357ce5b4c4867, kgergo5800, frauber3, Bacon Toast Guy, petrpetrov79, FlauN, re15revisited, Detective MAGNUM, Argus, M۞A۞G۞I۞C۞A۞L, Vera, slawuszka, Captain Snyder, Utong ng Yakuza, kty7410, Zeeko15, G-MON, Russia Greatest Love Machine, Biahwtrooper, VisorMachine™, Magmacow, Viz, [TRF] Commander Potato, raklo, Nom_carver, Yukon, [BB2] Faith, Gnome of War, TheBananaThief, Syntax