Gregory III did his best to reconcile monks, the church hierarchy, and common people to the agreement reached at Florence, but in vain.
In 1450, the tension in ecclesiastical circles grew so tense that Gregory III left his post and arrived in Rome in August 1451[2] (less than two years before the fall of Constantinople).
Pro-unionists in the Latin-occupied areas of Greece continued to consider him the legitimate patriarch of Constantinople.
He wrote two dissertations about the confutation of the works of the anti-unionist Bishop Mark of Ephesus and one on the provenance of the Holy Spirit.
Some of his letters have been preserved, while three further theological treatises, On the unleavened bread, On the Primacy of the Pope and On the Heavenly Beatitude, remain unpublished.