Gold stamped "925" is probably gold-plated sterling silver. The "925" is part of a precious metals hallmark system, but only silver is denoted "925," or 92.5 percent, pure.
Hallmarks are legends stamped on items made of precious metals, especially items large enough to have intrinsic value due to their precious-metal content. These hallmarks are composed of at least three symbols: the sponsor, the purity and the assay office approving the purity measurement. Any item that does not include these three symbols has no guarantee of purity.
The stamp "925" refers to purity, specifically the minimum parts per thousand of the item's precious metal content. Each precious metal uses only certain numerical standard measurements of purity. Gold does not use the number "925." It is not uncommon for the composition of "gold" jewelry to in fact be gold electroplate over hallmarked silver.