Gaheris was born to King Lot of Orkney and Queen Morgause, who was either the full sister or the half-sister of King Arthur, depending on the version. His brothers were Gawain, the eldest and the most celebrated knight of the Round Table; Agravaine, the sullen and treacherous second son; Gareth, the gentle and beloved youngest brother; and Mordred, the half-brother born of the incestuous union between Arthur and Morgause, the child who would one day destroy the kingdom. Gaheris grew up in the Orkney Islands, a harsh northern realm where the sea pounds the cliffs and the wind never stops, and he learned early that family loyalty was the only loyalty that mattered. When their father Lot rebelled against the young King Arthur and was slain by Pellinore at the Battle of Tarabel, the sons of Orkney swore vengeance. They would not rest until the man who had killed their father was dead, and they would not forgive anyone who stood in their way.

Before he was knighted, Gaheris served as squire to his brother Gawain, and it was in this role that he first appeared in Arthurian legend. When Arthur and Guinevere were married, Merlin sent Gawain on the quest of the white hart, a magical beast that had run through the wedding feast. Gaheris rode with him, and together they encountered Sir Allardin and other knights in combat. But the most telling moment of the quest came when Gawain, in the heat of battle, slew a lady who had rushed between him and her lord to save her husband's life. Gaheris, still a squire but already possessed of a sterner moral sense than his fiery brother, rebuked Gawain with words that would echo through the legend: a knight without mercy is without worship. Guinevere herself heard the tale and ordained that Gawain should forever after be the champion of all ladies, a penance that would shape his entire career. Gaheris's words, spoken from the position of a squire to a knight, revealed a young man who understood the cost of violence better than the warrior he served.

When Gaheris was knighted, he became one of the most active knights of the Round Table, though his fame was always eclipsed by Gawain's. He was defeated by Sir Turquine, the mighty knight who had captured sixty-four of Arthur's warriors and imprisoned them in his manor, and was rescued when Lancelot slew Turquine in single combat. Gaheris entered the manor, freed the prisoners, and bore witness to Lancelot's greatness, telling Arthur that Turquine was the strongest knight he had ever seen except for Lancelot himself. He was imprisoned in the Dolorous Prison near Dolorous Guard, the enchanted castle of Brandin of the Isles, and was rescued by Lancelot when the White Knight conquered the fortress. He was also imprisoned in the Valley of No Return with Aiglin of the Vales and Kay of Estral, and again it was Lancelot who freed him. These rescues established a bond of gratitude between Gaheris and Lancelot, a bond that would make their final encounter all the more tragic.

The darkest chapter of Gaheris's life was the murder of his own mother. Morgause had taken Sir Lamorak, the son of Pellinore, as her lover. Lamorak was one of the greatest knights in the world, second only to Lancelot and Tristram in prowess, and his affair with the queen of Orkney was a scandal that the sons could not bear. One day, Gaheris walked into his mother's chamber and found her in bed with Lamorak. In a fury of shame and rage, he drew his sword and struck off Morgause's head before Lamorak could even rise from the bed. The knight escaped, but the brothers hunted him down. They ambushed him, and while Gawain and the others held him, Mordred stabbed Lamorak in the back, a killing so cowardly that it stained the honour of the Orkney clan forever. When Arthur and the court discovered that Gaheris had killed his own mother, he was banished from Camelot, though he was later restored. The murder of Morgause and the ambush of Lamorak revealed the terrible flaw in the Orkney brothers: their loyalty to one another was absolute, but their sense of honour was twisted by vengeance, and they would kill anyone, even family, who shamed their name.

Gaheris's end came at the stake where Queen Guinevere was to be burned for her adultery with Lancelot. Agravaine and Mordred had plotted to trap the lovers, and when Lancelot was discovered in the queen's chamber, he killed Agravaine and several of his companions in his escape. Guinevere was condemned to death, and Arthur ordered the Orkney brothers to stand guard at the execution. Gawain refused, enraged that his king would burn the queen and furious at his brothers' treachery. But Gaheris and Gareth, though they had nothing to do with the plot and had refused to join it, were loyal to Arthur. They agreed to guard the queen, but they would not wear armour, for they would not stand armed against a fellow knight in what they knew was a wrongful execution. When Lancelot came with his men to rescue Guinevere, his wrath was so blind that he could not distinguish friend from foe. He cut down the unarmoured Gaheris and Gareth with a single sweep of his sword, not knowing who they were until it was too late. Lancelot was overcome with anguish when he learned what he had done, for he loved Gareth like a son. But Gawain's grief was beyond bearing. He swore eternal vengeance against Lancelot, and his fury drove Arthur to war against his greatest knight, the war that left the kingdom defenceless when Mordred rebelled. Gaheris was buried with Gawain in a tomb at Dover Castle, and the inscription on their grave marked the end of an era: the Orkney clan, which had produced the greatest knights and the darkest crimes in Arthurian legend, was extinguished by the very loyalty that had defined it.