GWYNLLYW or GUNLYU, latinised into Gundleus, and sometimes called Gwynllyw Filwr or The Warrior (6th cent.), Welsh saint, whose history, like that of all his class, is of more than doubtful authenticity, is said to have been the son of Glywys (Lat. Gliuusus), a South-Welsh king, whose genealogy up to Augustus Cæsar is given by the biographer of St. Cadoc (Rees, Cambro-British Saints, pp. 80-1). The same authority makes Gwynllyw's mother Guaul, a daughter of Ceredig, the son of Cunedda and the eponymous founder of Ceredigion. Gwynllyw had six brothers, and on his father's death the territory which he had ruled was divided among them all; but the younger recognised the overlordship of Gwynllyw, both as the oldest and worthiest of the sons of Glywys. They ruled among themselves over seven 'pagi' of the land of Morgan, part of which got to be called Gwenllwg, from Gwynllyw. The biographer of Gwynllyw dwells with rapture on the virtuous, prosperous, and peaceful rule of his hero, but the life of St. Cadoc represents him as violent and wicked, and the maintainer of robbers.
Gwynllyw is said to have married Gwladys, a daughter of the saintly Brychan of Brecheiniog. The would-be rationalisers of the lives of the Welsh saints profess that she must have been Brychan's granddaughter, to make the story fit in with their somewhat arbitrary and fanciful chronology. The 'Life of St. Cadoc' tells a picturesque story how Gwynllyw stole his wife from her father's court, but the wedding is a much more commonplace affair in the 'Life of Gwynllyw.' Their eldest son was Cadoc the Wise [q. v.], who became a famous saint. At last Cadoc's exhortations led Gwynllyw and Gwladys to give up their royal state and dwell in separate cells as hermits, performing the severest penances, and supporting themselves entirely by their own labour. They were frequently visited by St. Cadoc. The place of Gwynllyw's retirement was a certain hill above a river, a fruitful place, with a fair prospect of sea-coast, woods, and fields. There he built a church with boards and rods, and there he was buried. His last sickness was cheered by a visit from his son Cadoc and from Dubricius [q. v.], the bishop of Llandaff. The miracles worked at his tomb made it a famous place of pilgrimage. It is generally supposed to be the site of St. Woolos Church, the mother church of Newport-on-Usk. The feast day of St. Gwynllyw is 29 March, the reputed day of his death.
A less famous Gwynllyw or Gwynlleu was the descendant of Cunedda and the reputed founder of Nantcwnlle Church in Cardiganshire (Rees, Welsh Saints, p. 261). He is also to be distinguished from the female St. Gwenlliw, the daughter of Brynach or Brychan (ib. p. 142).[The chief authority for Gwynllyw's life is the Vita Sancti Gundlei Regis, printed (with an English translation) from the twelfth-century Cott. MS. Vesp. A. xiv., in W. J. Rees's Lives of the Cambro-British Saints, pp. 145-57 (Welsh MSS. Sec.) It has been collated with the thirteenth-century Cott. MS. Titus D. xxii. Other and often contradictory references are made in the Vita Sancti Cadoci, also published in Rees. A more critical edition of these lives is promised by Mr. Phillimore. There is another short life, plainly based on the Vita Gundlei (Cott. MS. Tib. E. 1, and Tanner MS. 15), printed in Capgrave's Nov. Leg. Angl. and the Bollandists' Acta Sanctorum, xxix March, iii. 784. See also Prof. R. Rees's Welsh Saints, p. 170; Dict. of Christian. Biography; Hardy's Descriptive Catalogue of Manuscript Materials, i. 87-9.]