Constantius, at Sirmium on March 15, 351, proclaims Gallus his coadjutor and eventual successor to handle problems in the east, and arranges Gallus's marriage to his sister Constantia.

Gallus, the second of three children of Julius Constantius, had escaped the slaughter that followed the death of Constantine in 337—his elder brother had been killed in 34—purportedly because he was suffering at the time from what was thought to be a fatal illness.

(Julian, the youngest brother, will mention years later that he and Gallus were told repeatedly that Constantius' actions had been the result of a combination of misinformation and pressure from the soldiery, and that the emperor had come to regret his actions, which, in his mind, he linked to his childlessness and to military misfortunes on the Persian front.)