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.
minediamond000, Afrodight, a66208841, shompolovdenis, Whet_Memes, Cipher, RileyGame.ru(xprints), Alucard, Sneaky Snake, Pantherlily, Korneliusz, [1.SKM] Platinfuchs, Swordsteve the legendary SSJGod, Chest, JAMBOR, Nanook, 상큼한병신, deoxzygames, Erika, Rap Thor, <HS>F.B.I. M!K! #RiP TF2, jdramirezelmo