Gluck's Brothers are a duo of presumably-fictional characters.
The two cruel, selfish brothers of Gluck, the virtuous protagonist of the fairy story The King of the Golden River, were supposedly changed into black rocks in punishment after falling into the Golden River in the process of trying to exploit its gift, with the King of the Golden River had reserved for the selfless Gluck.
Behind the scenes
Gluck's Brothers are mentioned in the 1958 Carl Barks story Uncle Scrooge and the Golden River. They are, of course, based on the two selfish brothers of Gluck who serve as the main antagonists of John Ruskin's real-life work The King of the Golden River, where they are referred to by the moniker of the "Black Brothers", with their real names given to be Hans and Schwartz.