Inevitably, both there and in Malory, Gaheris is killed alongside his other brother Gareth during Lancelot's rescue of Guinevere, the event that will lead to the fall of Arthur.

This character, a prince named Gwalchafed [Gwalhafed] or Gwalhauet [Gwalhavet] (Old Welsh for "Hawk of Summer") ap Gwayr or mab Gwyar, mentioned in Culhwch and Olwen,[7] is a likely common source for both Gaheris and Gareth, if Gawain was indeed derived from Gwalchmai.

[9] The names of Gaheris and Gareth (as used by Thomas Malory) have various similar forms in Old French works and the adventures ascribed to the brothers may be thus interchangeable and indistinguishable, as some writers even had them confused within the same manuscripts of a given text.

[11] In continental literature, Gaheris' name is first found as Gaheriet (Gaherïet) on the list of King Arthur's knights in the French poem Erec and Enide, written in the late 12th century by Chrétien de Troyes.

In Wolfram von Eschenbach's German poem Parzival, the figure of Gaheriet is represented by Gawain's cousin named Gaherjet (Gaherjêt).

[13] As Karjet (Karyet), he also appears in Ulrich von Zatzikhoven's Lanzelet, helping Lancelot rescue Guinevere from the abduction by King Valerin.

In the Prose Lancelot, Gaheris is described as valiant, agile, and handsome (even as "his right arm was longer than the left"), but reticent in speech and prone to excess when angered.

The teenage Gaheris, together with Gawain and Agravain, defects from Lot and aids Arthur in the early wars against the rebel kings as well as the Saxons (substituted by the Saracens in some English versions such as Arthour and Merlin), especially distinguishing himself in fighting against the latter.

The young knight then sets out in the quest to save Gawain and Morholt, during which he is twice attacked by his envious brother Agravain but soundly defeats him on each occasion.

More notorious is his beheading of his own mother, Queen Morgause, after catching her in flagrante delicto with Lamorak, Pellinore's handsome son and one of the greatest knights of Arthur.

[18] 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.

That 'new' Gaheris (Gaheres de Norgales) participates in the resulting civil war, fighting on Arthur and Gawain's side against Lancelot's followers.