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.
Braixy, Marek, Tree, KawanSousa2019, Jasonawesome367, Yankee, gwk, rmn1981, [DATA THE EXPUNGED], azamani0326, 980034, Wendigo, Darkus, stachu159, RSG