Complex sentence: One view is that he lived with Julian in Nicomedia under the care of Eusebius, the bishop of Nicomedia until 340, when Eusebius became bishop of Constantinople, which necessitated Gallus and Julian to follow him to that city.
Split into simpler sentences: One view is that he lived with Julian in Nicomedia under the care of Eusebius, who was the bishop of Nicomedia until 340. At which point, Eusebius was made bishop of Constantinople, which necessitated Gallus and Julian to follow him to that city.