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.
◢ k r i s ◤, cute.boyammar, 1Dogg12, Drink Bleach Fda Approves, Raibowdash-zhenyacase, re15revisited, danika204242305, Squeeze, Saysay, underswap sans, igorcurak, dmitriy422, Jayzoh, Freddy, T800-Model 101, justincreator, ytp123, ShadowSniperBoy7, [NT] FatHat, Adolf Hitler, ldecaux, Robert Fagot, Dolan Trump, muaz615, thirsha4242