What was the most significant communications achievement during the Gilded Age?
Telephones and newspaper chains
There is more than one answer to this question. The Gilded Age lasted from around 1873 (when Mark Twain named it thus) to 1911 (The Triangle Waistcoat Fire ended the party for plutocrat millionaires, who could no longer credibly pretend that they were the good guys for holding back the labor movement in America). During this time, two communications breakthroughs occurred almost simultaneously: the development of the telephone, and the growth of newspaper chains.
The telephone developed gradually. In 1874, Alexander Graham Bell developed and patented the first telephone, and by the end of the decade, about 40,000 of them were in use in the US. They didn't become commonplace in households for several more decades; like all new communications geegaws, they were prohibitively expensive. Today, you likely have one in your pocket or might be reading this on one.
Newspapers, by contrast, were dirt cheap during this period and cost only a few cents. Newspaper readership was a lot more common than telephone ownership, and newspapers tailored their contents to local interests and tastes. Reading the news in San Francisco was a very different experience from reading one in New York.
This changed under William Randolph Hearst, who acquired the San Francisco Daily Examiner in 1887 and the New York Journal in 1895, and he began Hearst's Chicago American in 1900. Other newspapers and a slate of magazines followed. For the first time, readers all over the country were getting the same news and editorial messages.
His rival, Joseph Pulitzer, owned a somewhat smaller chain, including the St. Louis Post Dispatch and the New York World.
Today, most newspapers, television stations and radio stations are owned by a small handful of media outlets. This is rooted in the Gilded Age machinations of William Randolph Hearst, one of the drivers of that period.