Manners--eating, dressing, talking--carry a heavy weight of meaning in the poem. How are they reflected in its form (the feast topos, the passages of effictio, or description, the dialogues)? What do they mean?
I'm always struck by the fact that King Arthur, at his New Year's feast, refuses to eat or be served until all in attendance had been served. That seems to me to be the height of hospitality and graciousness, bestowing great honor on each guest. Even when this outrageously gigantic green man comes riding into the banquet on his outrageously green horse, Arthur is the consummate host. He doesn't get rid of his unexpected guest but simply waits patiently to see what will transpire--offering his hospitality until he has reason not to do so.
In the Middle Ages, these manners and rules of etiquette were very important for knights and lords and ladies. The knights had to adhere to codes of honor, chivalry and bravery. Lords and ladies also had to adhere to similar rules. For example, when none of the knights step up the accept the challenge that the Green Knight issues to the Round Table, Sir Gawain rises to the occasion in order to prevent King Arthur from having to accept the challenge, which would have been a great insult to him. This would have certainly caused King Arthur to be very angry and to doubt his knights' loyalty to him.
Another example is when Gawain travels to meet the challenge of the Green Knight. He lodges with Lord and Lady Bertilak. He accepts a deal with Lord Bertilak that they will exchange what they receive during the day. Bertilak upholds his end of the bargain, but Gawain fails to uphold his by not revealing he received a green sash (supposedly for protection) from Lady Bertilak. He had been truthful with Lord Bertilak up until this point. Because he broke this agreement, he is cut on the neck by the Green Knight. He is spared death because he was truthful to the Green Knight the rest of the time.