Prince Gwrgan probably fled Ergyng when his inheritance was seized by Gwrfoddw Hen around AD 619. Perhaps he was out of favour with the latter's nephew, High-King Arthur; although the chronology doesn't quite fit. He appears in four charters in the Book of Llandaff during the episcopate of Bishops Euddogwy and Inabwy.
Little else is known of this monarch, despite his epithet, the Great. Overlordship of Glywysing & Gwent may, however, be indicated if he is identified with Gwrgan Frych (the Freckled) who features in the Life of St. Cadog. This powerful lord granted the saint half the fishing rights on the Rivers Usk and Neath in return for the great sword of King Rhun Hir of Gwynedd and a fine new horse with all the trappings. Of course, St. Cadog died some years before Gwrgan's reign, but the story may relate to a later agreement with the saint's successors at Llancarfan Abbey.
Gwrgan ruled until about AD 645 when his sons, Caradog and Morgan, should have inherited the throne of Ergyng. It appears, however, that instead, his son-in-law, King Meurig of Glywysing & Gwent managed to seize power in the name of his own son, Athrwys.