Why would anyone not in communion with the Catholic Church be designated a "Doctor of the Church" (a title previously bestowed upon defenders of doctrine)? Isn't it reasonable to wonder why Francis would name a new Doctor who is not in communion with those great defenders of Catholic orthodoxy who have already been designated Doctors of the Church, such as Athanasius, Basil the Great, Aquinas, Augustine and Teresa of Avila?
St. Thomas Aquinas (Doctor of the Church): "Clearly the person who accepts the Church as an infallible guide will believe whatever the Church teaches". Shouldn't believing whatever the Church teaches be a basic prerequisite for the title, "Doctor of the Church"?
St. Gregory was born in 950 AD. IIRC, at that time, there was only one Holy, Catholic and Apostolic Church.