Upton Sinclair was referring to the reception his 1906 novel The Jungle received. In his book, Sinclair attempted to show the incredible hardships faced by poor people in general, and recent immigrants in particular, as they tried to make a living in the United States in the late 1800s/early 1900s. The attitude at the time was that America was such a land of opportunity that if a person couldn't make a living it was because he or she was lazy, and just didn't work hard enough. The term "gilded age" sounds as if it should mean things were wonderful, but "gilding" is a very, very thin layer of something valuable on top of something worth considerably less. This is what is implied by using the term "gilded age"--that there was a very small percentage of the population that had it all, and who structured society in such a way as to keep it.
In The Jungle, Sinclair wrote about the experience of the Rudkus family who had recently immigrated from Lithuania, and now lived in Chicago. They find work in the meat-packing district, but are abused physically, economically, sexually, socially...there is no way for them to succeed, or even to survive. They are expendable, because there will be new, innocent immigrants to take their places. Sinclair was trying to show how horrible the conditions were for the poor, but the main points focused on by critics and the public were his descriptions of the disgusting conditions in the stockyards and slaughterhouses. There were no regulations governing how food was produced, and the meat was shown to be spoiled, diseased, and contaminated. This directly led to formation of the Food and Drug Administration (FDA).