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.
xX_GuMMYGuTZZ_Xx, Armor busters, xoraz, Nightwing, Axel.v (PrehisJAWric), Tamschi, George Costanza, Waiscrim Gaming, Beyond Nature, chrish5168, vielvladimir, Bron, Zekuua96, Gigahertz Dude, [bad decisions], Derwen_Llewelyn