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.
Moose2024, Romaschkin, Romaschkin, fftnb, matheussantanasantiago1.0, ◢ k r i s ◤, steel, Bravo2YTpl, NeOtIc, re15revisited, Cathy Car, TheOnlySpy, banditt_34, banditt_34, www.ariyanmcpe, The Jackal, Grimlockprime222, justincreator, justincreator, Biahwtrooper, UncleSHREDDER, UncleSHREDDER, VeGas GhOst, Elite