Gallus (1) Caesar, son of Julius Constantius (youngest brother of Constantine the Great) and his first wife Galla; born A.D. 325 at Massa Veternensis near Siena in Tuscany (Amm. xiv. 11, 27). In the general massacre of the younger branches of the imperial family on the death of Constantine in 337, two young brothers were alone preservedâ€”Gallus who was ill of a sickness which seemed likely to be mortal, and Julian a child of seven.
Both were brought up as Christians, and entered with apparent zeal into the externals of the Christian life. In 350 Gallus received the dignity of Caesar, which the childless Constantius bestowed upon him on succeeding to the sole government of the empire by the death of his brother Constans. In the West Constantius was distracted by the usurpation of Magnentius in Gaul, while in the East the Persians were a perpetual source of alarm. Gallus had to make a solemn oath upon the Gospels not to undertake anything against the rights of his cousin, who similarly pledged himself to Gallus. He received at the same time the strong-minded and unfeminine Constantina as his wife, and Lucilianus, the count of the East, as his general (Zos. 2, 45. Philost. iv. 1 refers to the oath between Constantius and Gallus; cf. Chron. Pasch. p. 540; Zonaras, xiii. 8).
The records of his short reign at Antioch come to us chiefly from Ammianus (lib. xiv.). They are almost entirely unfavourable to him. His defence of the frontier against the Persians was indeed successful (Zos. 3, 1; Philost. iii. 28, speaks strongly on this point), but his internal policy was disastrous.
Besides the report of his harsh and open misgovernment, accounts of secret treason meditated by him were conveyed to Constantius. The emperor, with his usual craft, sent an affectionate letter and desired his presence, as he wished to consult him on urgent public business (Amm. xiv. 11, 1). When he arrived at Petovio in Noricum, he was seized by the count Barbatio, deprived of his imperial insignia, and conveyed, with many protestations that his life was safe, to Flanon in Dalmatia, where he was closely guarded. The all-powerful eunuch Eusebius was then sent to interrogate him upon his various crimes. Gallus did not deny them, but blamed his wife. Constantius ordered his execution, which took place towards the close of 354.
His instruction had been Arian under the direction of Constantius, and he seems to have been influenced not a little by the Anomoean Aetius. This notorious man had been sent to him to be put to death as a heretic. Gallus spared him on the intercession of Leontius, bp. of Antioch, and became very friendly with him. According to Philostorgius, he made him his religious instructor, and attempted by his means to recall Julian to the faith, when he heard that he was wavering (Philost. H. E. iii. 27). There is no reason to doubt that the young Caesar was a zealous Christian after a sort, and that he was distressed by his brother's danger of apostasy.