Some of the earliest Arthurian stories are told in Latin chronicles written by British monks. One of these chronicles, written by William of Malmesbury in 1125 AD, is probably the first written mention of Sir Gawain. He is referred to as Walwen in the following translation:
"It is said by some that Walwen's body was cast up from a shipwreck after he had been wounded by his enemies, while others say that he was murdered by his fellow citizens at a public feast. And so the truth lies in doubt, though neither story would lessen the assertion of his fame."
It's interesting to see how the contents of this short passage are expressed centuries later in a much larger way. In Morte Darthur and other tellings, a significant episode is the attempted murder of Gawain with a poisoned apple at a feast. In these same stories, Gawain ultimately dies in a boat on the coast of Dover after being wounded by Lancelot. The iteration of the Arthurian legend from the middle ages until today is part of what makes these stories so compelling.