Known as Gildas Sapiens (the wise) and also as Gildas Historicus, after Bede who called him the "Historian of the Britons".
The presumed author of the 6th century work most commonly known as De Excidio Britanniae, "Concerning the Ruin of Britain"; the only surviving contemporary narrative history of 5th and 6th century Britain.
One says ‘presumed’ because the work is anonymous, the first to ascribe it to Gildas was Bede, who based a large part of his early history on the De Excidio. As the name ‘Gildas’ is apparently unusual for the time, this has led some to speculate that it was a pseudonym. Which would be perfectly understandable given that he accused several Romano-British kings of sundry serious crimes and is scarcely less complimentary about the Anglo-Saxons for whom the most mildest term of reference is that of nefandi nominis, that is 'of an abominable name'.
Two biographies of Gildas do exist. The first from the 11th century, Vita Gildae auctore monacho Ruiensi, "A life of Gildas by a monk of Rhuys" (Rhuys is or was a Breton abbey), the second in the 12th century Vita Gildae auctore Caradoco Lancarbanensi, "A life of Gildas by Caradoc of Llancarfan" (a Welshman). The validity of both have been challenged on the grounds of chronological and factual error; but for what it is worth they both agree on certain details, such as;
The only clue that the author himself gives is that he was born in the year of the siege of Mons Badonicus or the battle of Badon Hill, which is generally dated at around 500 AD.
That and other clues in his writings allow us to say only a few things for certain. Namely;
Gildas is also considered to be the author of the Poenitentiae, a monastic Penitential, as well as some fragments of letters discussing matters of church discipline.