It is thought that both Gratus and Eustasius were of Greek origin and that they received their education and ecclesiastical formation from the type of monastic foundation in Italy established by Eusebius of Vercelli, which was modeled on that of the Eastern cenobites. 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.