Antonius Donatus Gregorius - Source

You may've reviewed this webpage, but In case you haven't, it may be of some value.

https://www.historyfiles.co.uk/KingListsBritain/CymruDemetia.htm

WEBPAGE SOURCE CITATION: Information by Peter Kessler, with additional information by Edward Dawson, from The Landscape of King Arthur, Geoffrey Ashe, from The Four Branches of the Mabinogi, Will Parker, from Welsh Genealogies AD 300-1400, Peter Bartrum, from A History of Wales, John Davies, 1994, and from External Link: Ancient Welsh Studies.