He was born in Scotland to a noble family, but forsook it to be educated (we think) at a monastery in Wales under St. Illtud. He became known as a teacher, traveling around Britain and Ireland while preaching and setting up churches and monasteries.
He eventually went to Brittany to become a hermit, but his reputation drew followers longing for instruction. He built a monastery for them on the Rhuys Peninsula. (Pictured above is a chapel to St. Gildas in France.) This s where he wrote his less-than-complimentary history
His De Excidio et Conquestu Britanniae ("On the Ruin and Conquest of Britain") was a savage treatment in three parts of the people and rulers in the past and their sinful ways.
Part one explains recent British history, starting with the Roman conquest and leading up to his own time (He gives his birth year as the same in which the Battle of Mount Badon is fought, which is said by some to be 452CE; the Annales Cambriae give his death as 570; both these dates cannot be mutually trustworthy). Part two is a criticism of five kings, some of whom are clearly documented figures. Part three condemns the current state of the British clergy.
Part one gets the most attention from scholars of Arthurian literature. In it, he mentions Ambrosius Aurelianus as the figure who led the British against the invading Saxons and drove them out. This section also has the earliest reference to the Battle of Mount Badon (Mons Badonicus), the major engagement against the Saxons which decisively stopped their advance and occupation of Great Britain.
If "Arthur" fought at the Battle of Mount Badon, and Gildas was born that year, he would have been very young during much of the time that the "Arthur" figure flourished. For some reason, however—oral tradition?—later biographies of Gildas tie him more tightly to Arthurian Legend, adding some details to those legends for the first time. But that's a story for tomorrow.