To me The Green Knight atmospherically portrays a moving and genuine myth about the core of human nature. The adventurer (could be a female) volunteers for a dangerous and honourable quest, facing danger, tempations and challenges for the development of strength of character. In the end, he voluntarily offers his neck for the Green Knights blade. After having discarded attachment to the fear and attractions of everyday consciousness, he honourably fulfills the agreement. This is because he recognises that the fulfillment of his destiny requires that he voluntarily move beyond the wayward nature of his thoughts and emotions and find a more authentic motivation for living. This attitude is symbolically expressed by his renunciation of his head.
Supporting such an interpretation are mythologies worldwide and throughout human existence. This is very clearly explained in Joseph Campbell's books (for instance The Hero's Journey), and provides a vivid and apt metaphor for the lives human beings have led throughout time, and continue to lead in the 22nd century.