The role that King Bans of Brittany and King Bors of Gaul play in the rebellion against Arthur is that they were his allies in the battle of Bedgrayne.
King Bans, also known as the King of Benwick, was the father of Sir Hector de Maris and Lancelot. He was an ally of King Arthur and also the brother of King Bors.
King Bors was the king of Gaunnes or Gaul in the early period of King Arthur's reign. Both King Bans and Bors appeared in the 13th century Lancelot Grail cycle.