The "Gilded Age" in America occurs right between at the end of the Civil War (around 1878) and the Spanish American War (circa 1888-9). It is basically the American version of the Industrial Revolution that had already taken over England. We became the country of iron and steel and the production grew tremendously over this period.
This was also a time when transportation boomed in the form of railroads, which were being built everywhere. Not only is this time called "gilded" inferring that there is "metal" everywhere, but also because of the immense amount of millionaires who arose to lavish and quite opulent lifestyles (gilded=covered with gold) out of the investments on steel, oil, trains, and other industrial achievements. For example, the Rockefeller's became who they are (to this day) from this period of time, claiming perhaps the bragging rights of being "the" richest American last name.
Do not confuse this time period with Mark Twain's novel The Gilded Age. However, it is always advisable to read literature related to this time period so that you can see what people who witnessed it saw through their own eyes.