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.
SC | Tickle, puypuy57, carlitos95, 10081971z, karl15maerz, Inspector Norse, racethis, Sound_Extreme, Veggie, turQuoise, aw001k3930, meilor30, eXtreme^, ıllıllı $Ќяɨℓℓ€ж ıllıllı, FakeManfred, Jäger, FieldersNL, darknessshadowevil, [GEWP]HappyNewYearSuperCaotic, fredchaine134