Gold stamped with an "18 KGP" mark means that the item in question is not solid gold. It is simply another metal that has been plated with a gold alloy comprised of 18 parts gold to six parts of another metal. These others metals are typically silver or zinc.
Gold plating is used instead of solid gold because a solid piece of gold is exorbitantly expensive to use in any substantial quantity. Alloys are used instead of pure 24K gold for a number of reasons. In some cases, it is to increase the hardness of the metal. In other cases, it is to alter the color and appearance of the gold. White gold, for example, is typically gold mixed with silver, palladium, platinum or nickel.