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.
killerplayer62, joemari, ◢ k r i s ◤, alexshulski, cute.boyammar, Sirzech Lucifer, OSITO (TED), 1Dogg12, merritttrainboy, Drink Bleach Fda Approves, Raibowdash-zhenyacase, re15revisited, danika204242305, Squeeze, Saysay, underswap sans, igorcurak, dmitriy422, Jayzoh, Freddy, ♤Isaac Clarke, justincreator, ytp123, ShadowSniperBoy7, [NT] FatHat, Papa Stalin, ldecaux, Robert Fagot, Dolan Trump, muaz615, thirsha4242