He deserves credit for clarifying, in a Catholic sense, the position of the Armenian Church with regard to the Council of Chalcedon and for re-establishing its ties with the See of Rome more effectively than his predecessor Gregory II, asserting its juridical independence against the claims of the Byzantine emperors. To this end, he corresponded with Popes Honorius II, Innocent II, and Eugenius III. He took part, together with his brother, Bishop Nerses the Gracious, in the Latin Council of Antioch (1139) and of Jerusalem (1142). Innocent II, having received information and praise from his legate Albert of Ostia, addressed a letter to G. (25 Sept. 1141) congratulating him and inviting his brother, Bishop Nerses, to Rome for further discussion. The embassy of Gregory III arrived in Viterbo around 1144–45 and was received by Eugenius III. The Apostolic See was informed about certain controversies between Armenians and Greeks and about notable disciplinary matters.
Gregory was the first Armenian katholikós to receive the pallium. He left as his literary legacy his correspondence and hymnographic compositions, which were introduced into the liturgy.