Chivalry was the name the conduct code that governed the kingdoms in Europe during the Middle Age ( between 1170 and 1220). It defended important values as justice, humility, loyalty to the king, and courage.
The Green Knight respects these codes. The reason is that he announces to the enemy before attacking.
"Pick up your grim tool now And let us see how you handle it."
This is the fragment that shows us he respects the chivalric values.