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.
◀Jordane05▶, namegus008, guliks, tpPT #CSGOPoints.com, Anonymous, Iucoat, Neon Streaks, Singu, Vyland, Mr.Puel, ☣ Vexer Wor ☣, ☣ Vexer Wor ☣, ☣ Vexer Wor ☣, ✪[SPQR] @rtem-predator✪, Dr. Ermac Freeman, mister_slender2, HyperCookieHD, Atlas, Atlas, Graaicko