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.
Dayzbasher, Schrödinger, volumetreblebasspower, famtwook, [CRP] Chat Sanders, ShamelessSellout, emergencytriplenine, Alex., wwwww369, MrPikou, re15revisited, re15revisited, Bradley, Phantom Six, Dark_Knight, Saysay, kundavid041555, kedsopon111, jorgenieblas, Melon, decadent_uk100, Banya,, AWESOMEKID1214, Zarous magicdrop.ru, Zarous magicdrop.ru, MrWARIO (dead), 4lpha, ash, (╯°□°）╯ The Rage of Silent Jay, Big Boss, Soup, gloowb, Nerolat, Bhhenry, whatsup1827, NikoIsCool1, tabraz, Turtleey, [email protected], park smoke, park smoke, Mr. Marooned, Dr.Doctor, Vergil DmC, Jd, Soldier, andrewdbuilder, andrewdbuilder