Roman martyrology: In the city of Nisa right in Cappadocia, Gregory, bishop and brother of St. Basil the Great, was an admirable man for his life and doctrine, which, by confessing the true faith, was driven from his see by the Arian Emperor Valens.

Etymology:  English form of Latin Gregorius, meaning "watchful, alert".

SHORT BIOGRAPHY

With his friend Gregory of Nazianzus and his brother Basil The Great, he makes up the trio known as "The Three Cappadocians".  

Son of two saints, Basil and Emmilia; young Gregory was raised by his older brother, St. Basil the Great, and his sister, Macrina, in modern-day Turkey. He was educated in Athens, Greece. Influenced by the works of Origen and Plato.
Gregory's success in his studies suggested great things were ahead for him, after becoming a professor of rhetoric, he was persuaded to devote his learning and efforts to the Church. By then married to Theosebeia, Gregory went on to study for the priesthood and become ordained (this at a time when celibacy was not a matter of law for priests).

Disillusioned with his life as a teacher, he became a priest and hermit; his mother and sister already lived the monastic life.  He was elected Bishop of Nyssa (in Lower Armenia) in 372, a period of great tension over the Arian heresy, which denied the divinity of Christ. He also participated in the second ecumenical Council at Constantinople as a theologian and fought Arianism; reaffirming the decrees of the Council of Nicaea. The council called him, “Father of the Fathers” because he was widely venerated as the great pillar of orthodoxy and the great opponent of Arianism.

It was after the death of his beloved brother ,Basil, that Gregory really came into his own. His fine reputation stayed with him for the remainder of his life, but over the centuries it gradually declined as the authorship of his writings became less and less certain, until thanks to the work of scholars in the 20th century, his stature is once again appreciated.

Indeed, St. Gregory of Nyssa is not seen simply as a pillar of orthodoxy*, but as one of the great contributors to the mystical tradition in Christian spirituality and to monasticism itself. It is said that he died in 398 of natural causes.

*Orthodoxy is a word that can raise red flags in our minds. To some people it may connote rigid attitudes that make no room for honest differences of opinion. But it might just as well suggest something else: Faith that has settled deep in one’s bones. Gregory’s faith was like that. So deeply imbedded was his faith in Jesus that he knew the divinity that Arianism denied. When we resist something offered as truth without knowing exactly why, it may be because our faith has settled in our bones. (Annotation made by americancatholic.org)