What caused the development of labor unions and to what degree did labor unions succeed in their goals during the Gilded Age?
The need for working men to file grievances against an employer as a group.
The first formal union was the Knights of Labor whose mission was to get men in a particular skilled trade concessions from their employers. For example, steam locomotive engineers wanted safety improved on their engines (in the 19th steam locomotive blew up a lot).
The Knights of Labor, its successor the AFL and the IWW (Industrial Workers of the World) were largely unsuccessful in all strikes in the 19th century and well into the 20th century. In 1910 about 75% of all strike ended unsuccessfully. Striking workers were simply replaced and when that did not work the industry involved would simply wait them out. Few strikes lasted longer than 10 days.
That changed in 1912 when an IWW supported strike in Lawrence MA lasted 62 days and all the strikers' demands were met. Working men suddenly had a blueprint for success.