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.
ereny2793, Wanna Be Clorox, TheDovahkiinn, Layoursak_Sama, ninja60418, WalterKing, jan.c, tolf-dietram, angrysteroids, killer potATO, mq, RKOer2231, Sir CashewNuts, WoobyChicken, amirardeshir, Charles, alexisrn2004, ultraplush1987, Nayadi, balazs5001, AdriusBR, NorskMan123, Zahv, JigglyPuffSparkleFairy, MateuszNEXT