There is good reason why Mark Twain (and fellow writer Charles Dudley) would have referred to an entire age using the word "gilded."
The term "gilded" is defined by Dictionary.com in a manner that strikes at the heart of the fallacy of what many called the "gilded age."
[gilded] - having a pleasing or showy appearance that conceals something of little worth
The post-Civil War era may have been a time of plenty for some, but many, especially those in the South, had lost everything. Even so, in the North, economists recognized what became known as "robber barons," men who started with moderate wealth and turned it into massive fortunes upon the backs of those who worked for them. They were opposed to unions, which would have cost them more money, and while some people saw these industrialists building a new nation on the rubble of the Civil War's destruction, there can be no doubt that what was presented to public view was "colored" by propaganda.
Names like Rockefeller, Carnegie and Astor were men who built their empires in steel, the railroad, and finance.
Appearing in literature during the late 19th century, the Robber Baron thesis was popular until the 1940s. Matthew Josephson's The Robber Barons gave the term its most enduring expression. The theme was popular during the Great Depression amid public scorn for big business.
In terms of the concept of "the gilded age:"
In American history, the Gilded Age refers to the era of rapid economic and population growth in the United States during the post-Civil War and post-Reconstruction eras of the late 19th century. The term "Gilded Age" was coined by Mark Twain and Charles Dudley Warner in their 1873 book, The Gilded Age: A Tale of Today. The name refers to the process of gilding an object with a superficial layer of gold and is meant to make fun of ostentatious display while playing on the term "golden age."
A safe assumption, in light of this information, is that Twain, with his unerringly keen eye for seeing through the sham and façades of most questionable situations, saw what he referred to as the "gilded age," as nothing more than a "storefront" that prettily covered the disreputable business venture within.
While the moderately rich were getting richer, Twain saw the suffering that was being experienced by the less fortunate, and there were many. Twain was not known as a man to pull literary "punches," and he became the voice of the common man.
Twain refers to the sense of "gilded" in terms of an entire age because it was a time when there was much that was pleasing to the eye, like a shiny bauble appeals to a child; however, looking beneath the surface, it was easy to see the true "state of the union," by looking to those who were trying to rebuild their post-Civil War lives, and the "carpetbagger*" mentality displayed in people like "robber barons" who put on a good show, as long as one didn't "scratch the surface" to see the muck, poverty and suffering hidden by the "golden" cosmetic that pacified so many into a belief that life was good and getting better.
Source: carpetbagger - http://en.wikipedia.org/wiki/Carpetbagger