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.
samuel1062017, WE NEED COMMUNISM, SwiftGalaxy899, Ricowolf, сМаКуС & 35, DoNotFx, jan.hymr, MrDono, 1228256275, Reed Timmer, ♤Isaac Clarke, iamkaixa, vojtakarasek11, Mexican Budgie, eisenhower, Hds46, Қatka ♥, daniel1005