GUARD, KNIGHTS OF THE
Knights of the Watch

When Gawaine comes to the Rock of Canguin, Queen Ygerne commences her questions about Arthur's household by asking her guest if he is one of the knights of the guard [Owen's translation] or watch [Cline's translation], who have done many valiant deeds.

She does not mean the Round Table, for she proceeds to ask about that separately after Gawaine has answered that he is not a knight of the guard; he does admit to belonging to the Round Table, calling himself "neither the best nor the worst" of that company.

Unless the Knights of the Guard are to be equated with the Queen's Knights, I do not recall yet meeting them as a specific body anywhere else in Arthurian lore. Arthur's court must certainly have had its share of other knights, whether waiting and hoping to join the Round Table or content with lifelong service in less exalted but still essential places.