A stamp reading 18KGP means that the jewelry is gold plated, so an 18KGP white gold item would be plated with 18 karat white gold. The metal underneath the gold is usually copper or silver, but these can diffuse into the gold and tarnish the color.
Copper has a nickel-plated layer before adding the gold, while silver metals usually have layers of copper and then nickel. The nickel is a barrier metal that can help stop the primary metal from ruining the finish of the gold. If the barrier metal is not used, it can take only a few months to a few years before the gold is tarnished.
Karat is a term used to describe the purity of the metal and 18 karat gold is gold mixed with other metals, making it 75 percent pure. Each karat represents 1/24th of the gold's purity, or 4.1667 percent. Twenty-four karat gold is the purest, and 12 karat gold is 50 percent pure. Gold jewelry is usually sold at between 9 and 18 karat, and in the U.S. the minimum value for gold is 10. Gold is mixed with other metals in jewelry because gold as a metal can be damaged easily. Twenty-four karat gold is usually reserved for ceremonial or display use.