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.
ocEanManDiEdL0l, Dukalian Aldverse, nedzo, gabrielecogliati, ThePlayer263, assasin5678, dez nutz, [Dotka] Me Sentry, Seth Rolins, gabrielecogliati, StormFX, Ｍｒ．ｊｅｊｅ, SupaDope47, The Black mask, lan01290129, Teabag2001, kouptsov, Mr.Fizzlesticks, JoshTHETANK #MiloMVP, piplup529, Mank Demes (Can't be melted), Niebieska Szykość, Mike The Comrade