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.
Raibowdash-zhenyacase, re15revisited, danika204242305, Squeeze, Saysay, igorcurak, dmitriy422, Jayzoh, anapui37, lord of empire, justincreator, ytp123, (MLG)ShadowSniperBoy8418413, [NT] FatHat, Gabe Newell, ldecaux, Robert Fagot, Dolan Trump, muaz615, thirsha4242