When you see 18KGP on a piece of jewellery, it means that it has a thin layer of 18 karat gold on it. The thin plating is stuck to a base metal that is not as valuable.
One of the biggest problems with gold-plated jewellery is that the gold layer is so thin that it sometimes flakes off, revealing the base metal underneath. The difference between gold-filled and gold-plated is an important thing for people who buy gold to remember when they buy things. Gold-filled jewellery is not as valuable as solid gold, but it is better quality than gold-plated jewellery.