Men are forgetting as I speak to you; By her head sever’d in that awful drouth Of pity that drew Agravaine’s fell blow,
Guenevere's accusation suggest that Sir Gauwaine cannot claim moral superiority, as his own family history is fraught with similar transgression. This highlights the recurring theme of hypocrisy and flawed virtue among Arthurian knights. Furthermore, Guenevere is referring to the affair of Sir Gauwaine's mother, Morgause. She was killed by her son, Gaheris, when he discovered her relationship with Sir Lamorak. https://kingarthursknights.com/arthurian-characters/morgause/