Gluck was a (presumably fictional) male character of unknown species.
The main protagonist of the fairy story The King of the Golden River, Gluck was an exceptionally selfless young man. In thanks for his good deeds, a gnome king told him how to make a certain waterfall flow with gold. Being so unselfish, Gluck gave all the waters he could have used for the trick to thirsty travelers on the way, which turned out to be one last test at the end of which the King still gave Gluck the gold. Gluck then told the story to his two cruel, selfish brothers, who attempted to get the gold for themselves but fell into the fall and were transformed by the gnome's magic into a pair of rocks as black as their hearts.
Although the Golden River really existed, having been found in 1958 by Scrooge McDuck, Donald Duck and Huey, Dewey and Louie Duck, it is unclear if the fairy tale was literally true, or merely a fiction inspired by a real landmark. At the very least, the way the "magic trick" is described in the fairy story does not quite fit with what Huey, Dewey and Louie discovered.
Behind the scenes
Gluck is repeatedly mentioned in the 1958 Carl Barks story Uncle Scrooge and the Golden River. He is, of course, based on the Gluck who serves as the main protagonist of John Ruskin's real-life work The King of the Golden River.