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.
FemPyro♥, amirardeshir, amirardeshir, bradobedencio, sebastianestrelladiaz, gaben newell, absza15, sebastianestrelladiaz, Just丿A灬TALE, absza01, joecox1231, Wade(, ssh99o927, ssh99o927, [ISIS] Jimmy Sav, Lord Eoss, jw93859, Xoma-X4, ikosmid