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.
Jack5, misopsychia, sebastianestrelladiaz, Flynn Taggart, email@example.com, Thatguywithdalisp, TMB, amaximenko., erikfelix1999, XtremeTerminator4, BLλZING_DUST, Brad, Youtuber Bill Nye The Medic Guy