Gaheris is a Knight of the Round Table and a relative of King Arthur in Thomas Malory's Arthurian legend compilation Le Morte d'Arthur. He is the third son of one of Arthur's half-sisters, Queen Morga
Francis Arthur Fraser's illustration of Gaheris (right) as a squire to his brother Gawain
Galvagin (presumed Gwalchmai/Gawain) being followed by Galvariun (possibly Gwalchafed/Gaheriet) on the Italian Modena Archivolt (c. 1120–1240)
"Gerreet" fighting to free a captive knight in a 15th-century manuscript of Lancelot du Lac (BnF Fr. 111)
Gareth of Orkney is a Knight of the Round Table in the Le Morte d'Arthur version of the Arthurian legend and many modern works based on it. He is depicted as the youngest son of King Lot and the Queen
Gaheris
…of Arthur's half-sisters, Queen Morgause, and her husband King Lot of Orkney. Gaheris is a younger brother of Gawain and Agravain, an elder brother of Gareth, and a half-brother of Mordred. Malory's characters of Gaheris and Gareth both originate from the figure of Gawain's sole brother from the early Welsh…
Arthur Rackham, "How Beaumains defeated the Red Knight, and always the damosel spake many foul words unto him." From The Romance of King Arthur (1917), abridged from Malory's Morte d'Arthur by Alfred W. Pollard
"The lady Lyoness [...] had his [Gareth's] dwarf in examination." N. C. Wyeth's illustration of her and Melot in The Boy's King Arthur (1922)
Gareth and Lynette, a photo by Julia Margaret Cameron (1874)