18KGP on a piece of jewelry means that the item is gold-plated with a thin layer of 18 karat gold. The thin plating is bonded onto a less valuable base metal.
One of the main problems with gold-plated jewelry is that, because the layer of gold is so thin, it sometimes flakes off and exposes the base metal underneath the gold. An important point for gold buyers to keep in mind when they purchase items is the difference between gold-filled and gold-plated. Gold-filled jewelry is not as valuable as solid gold, but it is of a higher quality than gold-plated jewelry.