Arthur is mentioned long before Geoffrey of Monmouth, try the Historia
Brittonum, or the Welsh Annals, or Culwych and Olwen, or (as you say) the
Gododdin, or other early Welsh poetry, his legend was also in Brittany
before the writings of Geoffrey of Monmouth, making it difficult for
Lancelot to be the Breton Arthur, the source for this is the article by
Oliver Padel in, Cambrian Medieval Celtic Studies, 1995 on Arthur.
The early material for Arthur cannot then be compared to that of
Lancelot. Lancelot doesn't even get a mention in Geoffrey of Monmouth,
the greatest of all concocters of 'fiction' and seive of things Britonnic.
Mark Handley, Trinity Hall Cambridge.