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.

Gratus represented the bishop of Aosta, Eustasius, at this council,[2] signing the letter that the assembly sent to Pope Leo I the Great in order to affirm its condemnation of the heresy of Eutyches.

The year of his death is not known, but the day is: his tomb in the parochial church of Saint-Christophe bears an inscription that reads Hic requiescit in pace S. M. GRATUS EPS D P SUB D. VII ID.

During the Middle Ages, Gratus was invoked against a series of natural disasters: floods caused by the melting of the winter snows; drought; hailstones; fire; grasshoppers and moles that devastated the fields.

[5] In 1285, the Magna Legenda Sancti Grati, a fictitious and anachronistic account of his life, was composed by Jacques de Cours, canon of Aosta Cathedral, to celebrate the translation of the saint's relics.

In the twentieth century, the historian Aimé Pierre Frutaz demonstrated that the Magna Legenda Sancti Grati was an invented tale.

However, the tale had spurred the diffusion of Gratus' cult into Piedmont, Lombardy, Switzerland and Savoy, and provided the basis for the saint's attributes.