"24KGB" is short for 24-karat gold bonding. This is a technique in which base layers of 24-karat gold are covered with layers of 14- or 18-karat gold to create a more affordable replica.
Gold bonding generally starts with a base material such as bronze. The base material is then covered in layers of 24-karat gold, and these are then covered in layers of 14- or 18-karat gold. The resulting material is very similar in appearance, weight and flexibility to 14- or 18-karat gold but does not cost as much. Gold bonding may also be referred to as "gold filled" or "gold overlay."