How does Gawain break is promise to the lord?

Gawain breaks his promise to the lord in "Sir Gawain and the Green Knight" when he accepts a green girdle from Lady Bertilak, which she claims will protect him from harm. Although Gawain initially agrees to exchange whatever he receives during his stay with the lord, he conceals the girdle from him, prioritizing his own safety over honesty and loyalty. This act of deceit ultimately reveals Gawain's human fallibility and the complexities of chivalric ideals.