It really depends on the plate chiller. The manufacturer will state if it is compatible with a gravity fed system, some are and some are not. If it's not listed contact them and find out before purchase. They may give you details such as the amount of drop to function properly. Just because they use a garden hose doesn't mean it will work.
Are you suggesting that your cold water source in be gravity fed as well as the wort? This can't really work well as the water source goes in the bottom feed and the wort through the top so the two liquids pass opposite each other. At some point the pressure of the cold water source will reduce to the point it will no longer push up and through I would think.......Perhaps I'm Confused................