This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
jpstirad, Taika, UghZan, DeathGardenXVII, nightrainmusic, Dakota, ariano113, ariano113, DandyEndo, TheFlerker, Mr Andrew Togoret, King of Hate, jpstirad, jpstirad, razzan, activatorulvoltzengan, B1ts -> Going Back [adding], Selium, TheGuitarist-Ish, TrollekMolek15%, DJ-sona, skeon dev, Nexeleon, ELITE COMBINEXD, Killer Memestar AH, xyzzy, xyzzy, Broyan, Blacky Tiny Pyro, Ahmed Jeddi, nesguy229, Deluvas, Velox, Wheelchair Warrior, Wheelchair Warrior, 피아바수, kipper, Zenolisk, Mka0207, Paw83, FiLzO, StealthPaw , Hds46, CONRECON, Unknown