@blatham,
Sir Gawain and the Green Knight (in Middle English,
Sir Gawayn and þe Grene Knyȝt) is certainly an Arthurian romance, dated to the early 14th century. It has nothing to do, however, with the narrative canon established by Thomas Malory in his
Le Morte d'Arthur. written in the late 15th century, and printed and published by Caxton in 1485.
Sir Gawain and the Green Knight makes no reference to the death of Arthur, nor to his burial on the putative Isle of Avalon.
The entire Arthurian romance cycle was, and remains, the most fertile of literary traditions in Europe. The historical evidence is that no such British king ever existed. Gildas, a British monk who lived in Brittany and who asserts that he was born in Britain in the same year as the battle of Badon Hill does not say who commanded at that pivotal battle.
Gildas never mentions any King Arthur. Gildas was well known in his own day (late 5th century and early 6th century), and was well respected in church literary and historical circles. In his narrative,
De Excidio et Conquestu Britanniae (On the Ruin and Conquest of Britain) he refers to both people and events which are known to have existed or to have taken place from numerous church sources, for as well as anything from that era can be known. It is also worth noting at this point that the name Arthur does occur from that period--other than as an Irish name, the name of several prominent Irish invaders. (Many of those desperate to claim that this King Arthur did exist tie themselves in linguistic knots to attempt to derive the name from the British language of the 5th century.) It is also worth noting that at least one complete copy of the Gildas history has been in the possession of the church, ostensibly since the late 6th century, not long after he wrote it,
In the late 9th century, a manuscript was prepared by a monk thought to have been named Nennius (no proof of his existence is to be found--unlike Gildas). He is attributed as the author of
Historia Brittonum,
The History of Britain. There is no reference to either the author or the work before the mid-10th century.
In the basic narrative, Nennius never mentions any King Arthur. There is an "appendix" which is attributed to Nennius,
The Twelve Victories of Arthur.. No copy of either Nennius'
History or the appendix exists from earlier than the 14th century. I have an opinion on the origin of the "appendix," but won't rehearse it here.
In
Annales Cambriae (
The Annals of Wales) and the Welsh Triads, Arthur is mentioned. No copies of either document exist from any earlier than the 14th century.
The greatest Arthurian manuscript in Britain prior to Malory is
De gestis Britonum (
On the Deeds of the Britons), later popularized as
Historia regum Britanniae (
The History of the Kings of Britain), written by Geoffrey of Monmouth in the early 12th century. It is so wildly, extravagantly implausible as to be an embarrassment to historians. When it refers to historical events recorded in other sources, it almost invariably disagrees, and appears to be wholly unreliable. It was damned popular, though. It was so silly, historically speaking, that it doesn't matter how old any manuscript copies are.
Chrétien de Troyes writing later in the same century, and creator of Lancelot, may have used Geoffrey of Monmouth as a source. Marie de France wrote a series of short tales about the Arthurian knights, probably in the late 12th and early 13th centuries--and probably while living in England as they are written in Anglo-Norman. In both cases, the manuscripts from at or near the time of their composition are available (they were both "best sellers"), and no one considers them to be history, just popular entertainment.
Therefore, the Arthurian tales, including the burial of Arthur on the "Isle of Avalon," are certainly very important in medieval literature. History they ain't. It is my settled conviction, based on almost 50 years of reading (certainly not continuously, although very widely) is that King Arthur is only a creature of imagination. If anyone was a type for the King Arthur character, it would have been Ambrosius Aurelianus, an historically well-established individual. He is mentioned by both Gildas and Nennius. The probably spurious appendix of Nennius could be read as a description of the campaigns of Ambrosius Aurelianus against the Saxons.
The final irony in all of this is that Arthur is revered by the Anglo-Saxons, when if he had existed, he would have been he who dealt them their most stunning defeat. Even the Anglo-Saxon Chronicles, which don't mention Badon Hill, are silent about Saxon advances against the British for more than two generations. Gildas asserts that the Saxons were driven back for about that period of time--but, of course, Gildas don't know no steenking King Arthur.