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.
DanielAver, stepchenko-2016, Sabotinekes, ☣el chavon☣, gabimagloviti, bloodbathbowl, francescopaolo, ozike21, MrEggs0925, dorukkcr, [HR] Maximus, UnforgivenCaleb, vitaliy.derezer, lonely19682005, Reverend Jesse V92, Vox Maro, Vexcernite