I cannot find anything that mentions Sir Gawain and Sir Tristram being mentioned in any historical writings. This is what I found from the History Channel website. (I went to that site because I recently watched their show on the subject.)
We’ve all heard stories about King Arthur of Camelot, who according to medieval legend led British forces (including his trusted Knights of the Round Table) in battle against Saxon invaders in the early sixth century. But was King Arthur actually a real person, or simply a hero of Celtic mythology? Though debate has gone on for centuries, historians have been unable to confirm that Arthur really existed. He doesn’t appear in the only surviving contemporary source about the Saxon invasion, in which the Celtic monk Gildas wrote of a real-life battle at Mons Badonicus (Badon Hills) around 500 A.D. Several hundred years later, Arthur appears for the first time in the writings of a Welsh historian named Nennius, who gave a list of 12 battles the warrior king supposedly fought. All drawn from Welsh poetry, the battles took place in so many different times and places that it would have been impossible for one man to have participated in all of them.
www.history.com/news/ask-history/was-king-arthur-a-real-pers
