The decade was a period of exceptional economic expansion, and, in particular, of rapid wealth gains in New York City and Boston. The American empire of trade was at one of its zeniths, and cities were growing rapidly. While the same decade saw an explosion of immigration to the United States from less economically prosperous lands, and consequently little gaiety for the working classes, it was a period of vast wealth for a newly emergent "society set". The railroads, the agricultural depression of the Southern United States, and the dominance of the United States in South American markets and the Caribbean meant that industrialists of New England were doing very well.
The term itself began to be used in the 1920s and is believed to have been created by the artist Richard V. Culter, who first released a series of drawings in Life magazine entitled "the Gay Nineties" and later published a book of drawings with the same name. The high life of the "old money" families was well documented in the novels of, for example, Edith Wharton.
It was also the name of a nostalgic radio program in the 1930s, hosted by a prominent composer of popular songs of the 1890s, Joe Howard. In the 1920s through the 1940s, filmmakers had a nostalgic interest in the 1890s, as can be seen in the films The Naughty Nineties, She Done Him Wrong, Belle of the Nineties and The Nifty Nineties
The phrase has nothing to do with the term "gay" to refer to homosexuality, a usage that long postdates the phrase.