This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
Pozelei-Dilektor, hbn073, 幽冥樓閣の亡霊少女, dawe, sllkd005, calvinbrown789, Nightle ;3, soviet-eagle, Kendrick, Warff Woof!, JAN.HYMR, Hyper Nova, Patidinho, Sabun, Хороший суслик, Bobe, Vandarxx, 6077, Gaemi(게리인턴모집-, Dima_A, Icewine Rose, dmita.spasic, Mullins the Great