Gold-filled is more valuable than gold-plated. A piece of jewelry that is gold-filled is not solid gold, but it has a thick layer of real gold bonded to another metal. Since the piece is covered with real gold, it is virtually impossible to tell the difference between a solid gold piece of jewelry and a gold-filled piece of jewelry.
A gold plated piece has a thin layer of gold on the outside that is bonded to another metal. It will tarnish if not treated.