[NYer]

Why would anyone not in communion with the Catholic Church be designated a "Doctor of the Church"

St. Gregory was born in 950 AD. IIRC, at that time, there was only one Holy, Catholic and Apostolic Church.

[NRx]

I’m sorry, but that’s simply inaccurate. In the tenth century the schism with the non-Chalcedonians was already close to 400 years old. The Armenian Church was, and is, Miaphysite in its doctrine.