In his youth, when he was a pagan, Gradlon is said to have fallen deeply in love with a beautiful half-woman/half-fairy. Unfortunately however, he later offended this mysterious creature by converting to Christianity and seeking the counsel of St. Gwenole. The fairy fled from Gradlon across a torrential river, warning him not to follow. The brave Breton, however, plunged headlong into the swirling waters after her. The fairy was forced to save his life, but spurned him nevertheless and hated him all the more for having made her admit that she loved him still.

The so-called ' King' - possibly a governor if he existed - took quickly to the new religion. While out hunting, one day, he became separated from his entourage and found himself lost in the great Forest of Menez-Hom. Almost dead from exhaustion and hunger, he eventually stumbled across the hermitage of St. Cornetin (now built upon by Plomodiern). St. Corentin kept a marvellous fish from which he cut a slice to sustain him every day. The fish would be found miraculously whole again in the morning. The saint happily shared his meal with the Breton King and he was thus brought back to health. As a reward for his hospitality, Gradlon made St. Corentin the first Bishop of Cornouaille.

The King's first fairy-love returned, in later years, to possess the mind of Gradlon's daughter, Dahut, who thence became the most wicked of Breton women. She built a castle over the River Argent in the Huelgoat Forest, to which a different suitor was invited every night. Each was given a black mask to wear so he could not look upon Dahut's face. Then the revelries began. At the end of a thoroughly enjoyable evening, however, hidden springs would shoot out from the mask and strangle the wearer! Dahut would then have the body thrown into the nearby forest caves.

Gradlon's main power-base was Cornouaille and his capital was the beautiful city of Caer Ys which stood in, what is now, the Bay of Douarnenez. It was a low lying city with a large dyke to keep back the sea. Gradlon kept the key to the sluice gates hung around his neck at all times. One night, however, the evil Dahut stole the key while her father was sleeping and slipped away to open the gates in some sort of sadistic game. The raging sea immediately invaded the city. Gradlon awoke with barely enough time to jump on his horse and race ahead of the torrential waves. Spying his daughter in the streets, and being unaware of her crimes, he just managed to pull her up behind him. On they rode ahead of the waves until they reached higher ground where St.Gwenole was waiting. "Cast off the demon that is riding behind you!" he cried, but Gradlon did not understand the saint. So Gwendole pushed Dahut into the sea himself, and immediately the waves were calm once more.

Gradlon made his new capital at Corspotium (Quimper), where his statue can be seen in front of the Cathedral. It is said that the bells of the churches of Ys can still be heard on stormy nights at sea. Traditionally, upon Gradlon's death, he was succeeded by his son, Salomon I; followed by his grandson, Aldrien.

Clearly most of Gradlon's life is totally legendary