According to Geoffrey of Monmouth, Gawain’s father sent him to Rome as a boy to serve in the household of Pope Sulpicius, who dubbed him a knight. Sulpicius is also mentioned in the Latin romance De Ortu Waluuanii, as a friend of Gawain’s foster-father, Viamundus, but the allusion derives from Geoffrey and does not imply any separate tradition.

No such Pope ever existed. Tatlock and others have explained Sulpicius as a product of Geoffrey’s waywardness or imperfect memory. He picked up the name of a real pope, Simplicius, and modified it. Simplicius succeeded to the papacy in 468 and held office till 483. Geoffrey’s apparent reference to him is unhistorical, but it hints that background reading may have led him to passages in chronicles where Simplicius figured. This is in keeping with his evident awareness of the reign of the eastern emperor Leo I, six years of which (468-74) overlapped Simplicius’s pontificate.

Within the overlap lies the Gallic campaign of the British king called Riothamus. Use by Geoffrey of known sources mentioning this campaign could have supplied him with the pope’s name as well as the emperor’s. Sulpicius, or Simplicius, may thus be held to strengthen the likelihood that he did use them, or other documents of similar content.

Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
De Ortu Waluuanii Nepotis Arturi | Late 13th century