Gwen has to some extent a bond with the knights of Camelot. Sir Elyan is her brother, Leon was her childhood friend, with Lancelot there is a mutual respect, Gwaine is a good friend. To me the writers are establishing that Gwen is in a way connected with them all. Guinevere is the future queen of Camelot. So the knights are bound to pledge their loyalties to her and protect her when it is needed. It will not be because of her title and their duties, but out of their respect and love for Guinevere herself.
