Per PopSugar, the term “Gilded Age” originated from their satirical 1873 novel. In reference to the era of economic growth — which saw a boom in new industries and “new money” — the coauthors compared the time period to “gilding,” a technique that sees a thin, outer layer of gold applied atop a less precious metal. Think gold-plated jewelry; to the common eye, it appears to be genuine gold, but in reality, it’s nowhere near as valuable.

Source: https://www.distractify.com/p/is-the-gilded-age-based-on-mark-twain-book