St Gregory was born in Constantinople around 1296. The emperor hoped he would serve in government, but he chose instead to become a monk on Mount Athos. There he lived the life of the hesychasts, the monks of stillness, who prayed the Jesus Prayer and taught that the pure in heart can see the uncreated light that shone from Christ at His Transfiguration on Mount Tabor.
In the 1330s the scholar Barlaam of Calabria mocked these monks and their prayer, arguing that God cannot be known directly at all. St Gregory answered him in the work known as the Triads in Defence of the Holy Hesychasts. After years of dispute, in which he was for a time condemned and imprisoned, councils in Constantinople upheld his teaching, finally in 1351.
At the heart of that teaching is a distinction the Orthodox Church holds to this day. God's essence, what He is in Himself, is beyond all knowing. But God truly gives Himself in His energies, His grace and light and life, which are not created things but God Himself acting. That is how a human being can be united to God, the gift the Church calls theosis, without ceasing to be a creature.
In 1347 he was consecrated Archbishop of Thessaloniki, though the city did not receive him for some years, and he remained its archbishop until his death in 1359. The Church keeps his memory on November 14 and again on the second Sunday of Great Lent, which bears his name.