The Catholic Encyclopedia warns that the two medieval lives of Gildas are confused, but harmonizes them this way. He was born by the Clyde to a noble British family and educated in Wales under St. Iltut, alongside St. Samson. He became a monk and went to Ireland, where he was ordained priest and built churches and monasteries.

He is said to have made a pilgrimage to Rome. On the way home his love of solitude led him to the island of Houat off Brittany. The Bretons found him and persuaded him to found a monastery at Rhuys. There he wrote his severe letter rebuking five British kings and the lazy clergy of his day. He is counted the earliest British historian. He died at Houat in 570.

S. Gildas, surnamed the Wise, was born in North-Britain near the Clyde, in the kingdom of which Dumbarton was the capital. His father was of princely birth; his eldest son, Howel, was a great warrior, who, succeeding his father in the principality, was slain by King Arthur. Mailoc, another son, was brought up to the religious life.

Surnamed the Wise; b. about 516; d. at Houat, Brittany, 570. Sometimes he is called "Badonicus" because, as he tells us, his birth took place the year the Britons gained a famous victory over the Saxons at Mount Badon, near Bath, Somersetshire (493 or 516). The biographies of Gildas exist — one written by an unknown Breton monk of the Abbey of Rhuys in the eleventh century, the other by Caradoc, a Welshman in the twelfth century. Both biographies contain unchronological and misleading statements, which have led some critics to reject the lives as altogether valueless. Ussher, Ware, Bale, Pits, and Colgan endeavour to adjust the discrepancies by contending that there were at least two saints named Gildas, hence their invention of such distinctive surnames as "Albanicus", "Badonicus", "Hibernicus", "Historicus", etc. The more general opinion, however, adopted by Lanigan, Leland, Healy, Stingfleet, Mabilon, Bollandus, and O'Hanlon, is that there was but one St. Gildas. The discrepancies may be accounted for by the fact that the lives were drawn up in separate countries, and several centuries after the saint existed. As to Caradoc's statement that Gildas died at Glastonbury, O'Hanlon remarks that Glastonbury appropriated more saints than Gildas (Lives of Irish Saints, I, 493).

Both narratives agree in several striking details, and may thus be harmonized: Gildas was born in Scotland on the banks of the Clyde (possibly at Dumbarton), of a noble British family. His father's name was Cau or Nau; his brother's, Huel or Cuil. He was educated in Wales under St. Iltut, and was a companion of St. Samson and St. Peter of Léon. Having embraced the monastic state, he passed over to Ireland, where he was advanced to the priesthood. He is said to have lived some time in Armagh, and then to have crossed to North Britain, his teaching there being confirmed by miracles. On his return to Ireland, at the invitation of King Ainmire, he strengthened the faith of many, and built monasteries and churches. The Irish annalists associate him with David and Cadoc in giving a special liturgy or Mass to the second order of Irish saints. He is said to have made a pilgrimage to Rome. On the homeward journey his love of so!itude caused him to retire to the Isle of Houat, off Brittany, where he lived a life of prayer, study and austerity. His place of retreat having become known, the Bretons induced him to establish a monastery at Rhuys on the mainland whither multitudes flocked (Marius Sepet, "St. Gildas de Rhuys", Paris, s.d.). It was at Rhuys he wrote his famous epistle to the British kings. His relics [the bodily remains of a saint, or objects they used, kept with honor] were venerated there till the tenth century, when they were carried for safety into Berry. In the eighteenth century they were said to be preserved in the cathedral of Vannes. He is the patron of several churches and monasteries in Brittany and elsewhere. His feast is locally observed on 29 January; another feast, 11 May, commemorates the translation [the solemn moving of a saint’s relics to a new shrine] of his relics.

The authentic work of St. Gildas, "De excidio Britannae liber querulus", is now usually divided into three parts: (1) The preface; (2) A sketch of British history from the Roman invasion to his own time; (3) An epistle of severe invective addressed to five petty British kings — Constantine, Vortipor, Cyneglas, Cynan, and Maelgwn. In the same epistle he addresses and rebukes the clergy whom he accuses of sloth and simony. His writings are clearly the work of a man of no ordinary culture and sanctity, and indicate that the author was thoroughly acquainted with the Sacred Scriptures.