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.
wwwww369, re15revisited, re15revisited, Xx_Shadow_xX, Private Dale, Dark_Knight, Saysay, kundavid041555, kedsopon111, jorgenieblas, decadent_uk100, Banya,, AWESOMEKID1214, Zarous, Zarous, MrWARIO (dead), 4lpha, Viz, (╯°□°）╯ The Rage of Silent Jay, Big Boss, Soup, gloowb, Nerolat, Bhhenry, whatsup1827, tabraz, Turtleey, [email protected], Konec, Konec, Mr. Marooned, Vergil DmC, jdramirezelmo, Soldier, andrewdbuilder, andrewdbuilder