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.
balazs5001, TheSrSky, Tom, ShunKiske, ▲ FaZe 2.0 ▲ Rainbow, wwwddfafff.aa, DonDisco, bautista13angelica, peaw._2012, ted_rv, master, Neksias, DONET, You Spoony Bard!, Turglar, Reverend Jesse V92, Reverend Jesse V92, Lambda Core, raygun gothic, spitey