Caradoc tells a story of how Gildas intervened between King Arthur and a certain King Melwas of the 'Summer Country' who had abducted Guinevere and brought her to his stronghold at Glastonbury, where Arthur soon arrived to besiege him. However, the peacemaking saint persuaded Melwas to release Guinevere and the two kings made peace. Caradog also says that the brothers of Gildas rose up against Arthur, refusing to acknowledge him as their lord. Arthur pursued Huail ap Caw, the eldest brother, and killed him. Gildas was preaching in Armagh in Ireland, at the time, and he was grieved by the news.
Gildas is credited with a hymn called the Lorica, or Breastplate, a prayer to be delivered from evil, which contains interesting specimens of Hiberno-Latin. A proverb is also attributed to Gildas mab y Gaw in the 'Englynion y Clyweid' in Llanstephan MS. 27.
In Bonedd y Saint, Gildas is recorded as having three sons and a daughter. Gwynnog ap Gildas and Noethon ap Gildas are named in the earliest tracts, together with their sister Dolgar. Another son, Tydech, is named in a later document. The unreliable Iolo Morganwg adds Saint Cenydd to the list.
Concerning her obstinacy, subjection and rebellion, about her second subjection and harsh servitude; concerning religion, of persecution, the holy martyrs, many heresies, of tyrants, of two plundering races, concerning the defense and a further devastation, of a second vengeance and a third devastation, concerning hunger, of the letter to Agitius [usually identified with the patrician Aëtius], of victory, of crimes, of enemies suddenly announced, a memorable plague, a council, an enemy more savage than the first, the subversion of cities, concerning those whose survived, and concerning the final victory of our country that has been granted to our time by the will of God.In the second part, opening with the assertion "Britain has kings, yet they are tyrants; it has judges, yet they are undutiful", Gildas addresses the lives and actions of five contemporary rulers: Constantine of Dumnonia, Aurelius Caninus, Vortiporius of the Demetae (now called Dyfed), Cuneglasus apparently of 'the Bear's Home' (possibly 'the Bear's Stronghold' — Dinarth at Llandrillo-yn-Rhôs near Llandudno), and lastly Maglocunus or Maelgwn. Without exception, Gildas declares each of these rulers cruel, rapacious, and living a life of sin.
The third part begins with the words, "Britain has priests, but they are fools; numerous ministers, but they are shameless; clerics, but they are wily plunderers." Gildas continues his jeremiad against the clergy of his age, but does not explicitly mention any names in this section, and so does not cast any light on the history of the Christian church in this period.
Gildas's work is of great importance to historians, because although it is not intended primarily as history, it is almost the only surviving source written by a near-contemporary of British events in the fifth and sixth centuries. The usual date that has been given for the composition of the work is some time in the 540s, but it is now regarded as quite possibly earlier, in the first quarter of the sixth century, or even before that.
The student must remember that Gildas' intent in his writing is to preach to his contemporaries after the manner of an old testament prophet, not to write an account for posterity: while Gildas offers one of the first descriptions of the Hadrian's Wall -- albeit highly historically inaccurate -- he also omits details where they do not contribute to his message. Nonetheless, it remains an important work for not only Medieval but English history for being one of the few works written in Britain to survive from the sixth century.
In De Excidio et Conquestu Britanniae, Gildas mentions that the year of his birth was the same year that the Battle of Mons Badonicus took place in. The Annales Cambriae gives the year of his death as 570; however the Annals of Tigernach date his death to 569.
Gildas's treatise was first published in 1525 by Polydore Vergil, but with many avowed alterations and omissions. In 1568 John Josseline, secretary to Archbishop Parker, issued a new edition of it more in conformity with manuscript authority; and in 1691 a still more carefully revised edition appeared at Oxford by Thomas Gale. It was frequently reprinted on the Continent during the 16th century, and once or twice since. The next English edition, described by August Potthast as editio pessima, was that published by the English Historical Society in 1838, and edited by the Rev. J. Stevenson. The text of Gildas founded on Gale's edition collated with two other MSS, with elaborate introductions, is included in the Monumenta Historica Britannica. Another edition is in Arthur West Haddan and Will Stubbs, Councils and ecclesiastical documents relating to Great Britain and Ireland (Oxford, 1869); the latest edition is that by Theodor Mommsen in Monumenta Germaniae Historica auct. antiq. xiii. (Chronica min. iii.), 1894.
In the later Old English period, Gildas's writing provides a major model for Alcuin's treatment of the Viking invasions, in particular his letters relating to the sack of Lindisfarne in 793. The invocation of Gildas as a historical example serves to suggest the idea of moral and religious reform as a remedy for the invasions. Likewise, Wulfstan of York draws on Gildas to make a similar point in his sermons, particularly in the Sermo Lupi ad Anglos.