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.
kitkitlei92, summedzebra406.gaming, marcelo.ubillus, Gamer Snake, JennyS•ßaL¡§øπG≈., Nopity, gb930323, fazafreeman, romanoromano57, 1228256275, BaLkAn GaMeS, busermanov5, milan7943, villa.andrea2002, Aweriy, Dude, [IG-SM] HerrSpray, Max.exe, Scar, Dr. Ermac Freeman, plutonic, Daniel the Painkiller, TheriDK, The Big Shell, FSM, nadia.efriyanti, Hds46, hjmen32