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.
A Real Human Bean, Imperial Soldier, hogr3, Korneliusz, jeffthekiller96, spartan117chile, RAMPAGE, MinderiaYoutuber, Edmond, Zaku II (laggy addon collector), Minimole the Lorax, Hirasmous, Rusky Husky, Alton Brown, Rawr, Pancakes 4.0