Gregorios 31 was bishop of Amastris in Paphlagonia; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at all eight sessions, from 24 September to 23 November 787: Mansi XII 998-XIII 486. In the lists of those present at the first and seventh sessions he is styled Γρηγορίου ἐπισκόπου Ἀμάστρης: Mansi XII 998, XIII 369. Elsewhere he is styled Γρηγόριος ὁ ὁσιώτατος ἐπίσκοπος Ἀμάστριδος or similar: Mansi XII 1099, XIII 145. At the second session he supported the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1099. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 145. His name is missing from the list of those who subscribed the statement of the faith adopted by the Council at the seventh session but he was present at the session and the omission is probably a textual error; perhaps it was in a lacuna at Mansi XIII 392.

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