It is likely that Gaheris and Gareth were actually the same character in origin, as their names in French sources are easy to confuse Guerrehet and Gaheriet, 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 is a likely source for Gaheris and Gareth, if Gawain was indeed derived from Gwalchmai.
Several of his adventures are narrated in the Lancelot-Grail. In the Prose Lancelot, Gaheris is described as valiant, agile, and handsome, but reticent in speech and prone to excess when angered. As such, he "was the least well spoken of all his peers." In the Vulgate Merlin, he is described as the best warrior among the brothers, and the Prose Tristan notes him as a far better knight than Gawain but nevertheless all the siblings are knighted at once. He is, however, the first of the Orkney clan to be knighted by King Arthur in the Post-Vulgate Cycle. When Gaheris is given flowers sent by the Queen of the Fairy Isle, it is prophesied that he would surpass in goodness and valor all the Knights of the Round Table save for two were it not for the death of his mother which Gaheris will cause through his sin. The young knight sets out in quest of Gawain and Morholt, during which he is twice attacked by his envious brother Agravain but soundly defeats him on each occasion, and eventually rescues both Gawain and Morholt, later accompanying the latter to Ireland. In Thomas Malory's Le Morte d'Arthur, based on the Post-Vulgate, Gaheris is at first a squire to his elder brother Gawain, whose fiery temper he helps moderate, before being knighted himself. Gaheris later marries the sister of his brother Gareth's wife Lyonors, the haughty damsel Lynette. The Lancelot and the Mort Artu sections of the Lancelot-Grail cycle differ in their characterisation of Gaheris. In the Lancelot, his younger brother Gareth is Gawain's most cherished brother. In the Mort Artu, Gaheris is represented as the most cherished, and his death anguishes Gawain profoundly. In the Post-Vulgate tradition, Gaheris participates in the revenge killing of King Pellinore, the slayer of his father King Lot. More notorious is his beheading of his own mother Morgause after catching her in flagrante delicto with Lamorak, Pellinore's handsome son and one of the greatest knights of Arthur. Lamorak meanwhile escapes, but is later hunted down by all of the Orkney brothers except Gareth, believing him to be their mother's murderer, and they fight him until Mordred stabs him in the back; the act that is deemed cowardly and a blot on their honour. When Arthur discovers that Gaheris is Morgause's killer, he is banished from court. Gaheris is then himself nearly beheaded in revenge for their mother's death by Mordred and Agravain when Gareth convinces Gawain to order them to stop. Despite being exiled, Gaheris appears later in the narrative as a one-time companion of Percival on the Grail Quest. His death during Lancelot's rescue of Queen Guinevere from being burned at the stake is related in the Mort Artu'', the final volume of the Vulgate and Post-Vulgate prose cycles. While Gawain and Gareth will have nothing to do with Agravain and Mordred's plot to entrap Lancelot and Guinevere, Arthur asks the all brothers of Mordred to help guard the queen's execution. Gaheris and Gareth reluctantly agree, though Gawain refuses. When Lancelot rushes to save the woman whom he loves, he cuts down the Orkney princes. The surviving brother Gawain's fury is terrible and the resulting feud leads to the destruction of Arthur's kingdom.
Gaheris of Carhaix
Gaheris, the brother of Gawain, should not be confused with a different character in the Mort Artu, Gaheris of Carhaix, brother of Mador de la Porte. This Gaheris dies from eating a poisoned fruit that was destined for Gawain by the knight named Avarlan and was offered to him unknowingly by Guinevere. Previously, in the Vulgate Lancelot, Gawain had saved him from Galehaut, and the White Knight rescued him from the Dolorous Prison near Dolorous Gard and then again from the Vale of No Return. This episode is also included in Stanzaic Morte Arthur and in Le Morte d'Arthur, however in these texts the victim is, respectively, either an unnamed visiting Scottish knight or one Patrise of Ireland. The story might have been inspired by account of the poisoning death of Walwen as told in the chronicleGesta Regum Anglorum.
In modern stories
T. H. White's The Once and Future King attributes the act of matricide to Agravaine instead of Gaheris. White gives his own individual interpretation to the story, depicting Agravaine as having an unhealthy love/lust obsession for his own mother, and repeatedly describes Gaheris as "dull" or "dull-witted".
In the film First Knight, Gaheris is portrayed by Alexis Denisof.
In Gerald Morris' The Squire's Tales, Gaheris is one of the main heroes, a witty, quietly brave man who prefers agriculture to sword fighting.
