Gregory XII was a pope who served as the head of the Roman Catholic Church from 1406 to 1415. Synonyms for Gregory XII include Angelo Correr, Baldassare Cossa, and John XXIII. Angelo Correr was the name of Gregory XII before he became pope. Baldassare Cossa was his rival, who was elected as a rival pope during the Western Schism. John XXIII is also a synonym for Gregory XII because he was elected as an antipope during the Western Schism and took on the name John XXIII. Despite the confusion and turmoil during the Western Schism, Gregory XII remained faithful to the papacy until he resigned in 1415.