In Thomas Malory's Le Morte d'Arthur, Gaheris is squire to his elder brother Gawain before being knighted himself, and helped moderate Gawain's fiery temper. He participates in the murders of King Pellinore, his father's slayer, and Sir Lamorak, Pellinore's son and the lover of Gaheris' mother. More notorious is his murder of his own mother, Morgause, after catching her in flagrante delicto with the youthful and handsome Lamorak. Gaheris beheads his mother but Lamorak escapes, to be hunted down by all the brothers except Gareth, and Mordred stabs him in the back. Since in Malory Lamorak is greater than any knight except Lancelot and Tristram, this act of revenge is deemed cowardly and a blot on the Orkney brothers' honour. When Arthur and the brothers discover Gaheris is Morgause' slayer, he is ejected from court.
Gaheris is killed accidentally by Lancelot during the rescue of Guinevere. Gaheris will have nothing to do with Agravain and Mordred's plot to entrap Lancelot and Guinevere, and when Arthur asks the brothers to help guard the Queen's execution, Gaheris and Gareth reluctantly agree, though Gawain refuses. They decline to wear armor, and Lancelot, unable to distinguish friend from foe in his blind wrath, cuts down the unarmored princes. Gawain's fury at this outrage is terrible and the resulting feud largely destroys the Round Table.
Gaheris is often little more than a supporting character to his brothers Gawain and Gareth, with the murder of Morgause an odd exception, leading modern authors such as T. H. White to give the act to Agravaine instead. He even marries the sister of Gareth's wife Lyonors, the haughty damsel Lynette. It is likely Gaheris and Gareth were the same character in origin, as their names in French sources, Guerrehes (Gaheris, usually) and Gaheriet (Gareth, usually), are easy to confuse and adventures ascribed to the brothers are often interchangeable. Furthermore, only one brother is ever named for Gwalchmai ap Gwyar, the character from Welsh mythology traditionally identified with Gawain. This character, Gwalchafed, is a likely source for Gaheris and Gareth if Gawain did indeed derive from Gwalchmai.