 He is known to have signed the acts of the synod of Milan in AD 451 as a priest.Gratus represented the bishop of Aosta, Eustasius, at this council, signing the letter that the assembly sent to Pope Leo I the Great in order to affirm its condemnation of the heresy of Eutyches.Gratus became bishop of Aosta sometime after 451, and presided over the translation of various relics in the city around 470, including those of Saint Innocent, one of the martyrs of the Theban Legion.