Gaheris was a member of the Knights of the Round Table, led by King Arthur, holding the 6th seat. He is the brother of Gawain, Agravain and Gareth, as well as a cousin of King Arthur. He and Gareth were killed during Lancelot's attempt to save Guinevere.
Gaheris and the other Knights of the Round Table, apart from Bedivere and Galahad, are summoned by the Lion King. The Lion King tells them to decide between joining her on her "Holy Selection" or fighting against her, giving them a half-a-day to decide. Gaheris resolved himself to part with his siblings, deciding to oppose the Lion King unlike them.
As he ultimately chose to oppose the Lion King, he fought and perished when fighting against his fellow knights and siblings.
- ↑ Camelot/Zero translated at Beast's Lair
 Fate/Apocrypha material - Encyclopedia: Knights of the Round Table [Organization], p.156-157 [T]
Knights of the Round Table [Organization]
The general term for the knights who left their names in King Arthur’s legend. They are called the Knight of the Round Table because they gathered around a round table along with King Arthur. The knights sitting around the table were all regarded as equal to one another, including the King himself, and they were admired throughout the country. Though the number of them varies according to each story, the standard pattern of thirteen knights is accepted in “Fate/stay night”.
In the “Apocrypha” world, the separate splinters from the Round Table serve as summoning catalysts for Heroic Spirits. From among eleven of the knights (with King Arthur and Galahad excluded), the one most compatible to the Master is summoned through it. Like an S-rare definite ticket. Whether it be someone like Lancelot, Gawain, Tristan or Percival, no matter who comes, you truly get the cream of the crop. During these current times where subspecies Holy Grail Wars are popular, they appear in the market at high prices. Naturally, there are many frauds and swindlers. It’d be nice if a certain oil magnate got swindled.