What happens at king arthur's court that shows the green knight's superhuman qualities?

When the Green Knight presents his challenge, King Arthur's knights are terrified. They sit there "still, now/All who followed Arthur, noble/And knave" (301-303). None of them accept its challenge because they know it is a death sentence. When Arthur rises to take the blow, though, Gawain is spurred into action out or loyalty for his kind and a desire that he die in Arthur's place.