Hildegard of Bingen
Etienne, Bishop of Liège, composed the first Gregorian chants that were used. The date he wrote them is not recorded, but he lived from 850 to 920 AD, and they are believed to have been written in the 10th century, so you have twenty years to play with.
Pope Saint Gregory the Great (would have been Pope Gregory the First) did not create Chant or Plainsong, but gathered it together and promoted it:from the Saints website:Elected 64th Pope by unanimous acclamation on 3 September 590, the first monk to be chosen. Sent Saint Augustine of Canterbury and a company of monks to evangelize England, and other missionaries to France, Spain, and Africa. Collected the melodies and plain chant so associated with him that they are now known as Gregorian Chants. One of the four great Doctors of the Latin Church. Wrote seminal works on the Mass and Divine Office, several of them dictated to his secretary, Saint Peter the Deacon.
Gregory Maguire wrote Confessions of an Ugly Stepsister.
Winnie Holzman wrote the book based on the musical. Gregory Maguire wrote the book that inspired the musical.
Gregory and Stalin are two separate people. Joseph Stalin was the ruthless leader of Soviet Russia and the Soviet Union. Paul Gregory is a writer who wrote a book about Stalin.
Tim Rice and Elton John composed the songs, while Lebo M wrote the African chants. Also, Hans Zimmer composed the score.
Gregory Maguire wrote the book Wicked: The Life and Times of the Wicked Witch of the West. Later, Stephen Schwartz wrote the music and Winnie Holzman wrote the book for the musical. :)