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.
ThatGuy8768, 3543450398, thedarko190, furiya6918, Ruliv, RealN96, QTG坑, Rhode, orcclans, SaikouDuellist, Purple_Guy_Ita, SoulsofDeath, Shadowfrost2134, StormFX, dismalequation, Santatune, KepperOfFish756 A.K.A alexgamin, Мяско:3, Sonicteam998, KEK | DankMeme, omy829, ILVIERA, DarkNezZ, malikjustice1, teh_sunrise, Death Gun