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.
CBT Master, 2241586939, Batuhan, Sordadinho, Sunder, Kotoshmetik, ᠌ ᠌, JSwannyGaming148, facoma4, ghh455, aoomsin_08, Pray For Glaz, bogosbrendon, i am a traitor, bobtheafk, deljavlada, sputt, sputt, ulicnikk, MordecaL70, Brains Collector, Brains Collector, Brains Collector, DyaMetR, xDShot, 76561198106129502