What was the reaction of Camelot when the Green Knight arrived? What was outstanding about his appearance?

The court is stunned by his appearance in the hall, as well as his physical appearance.

The Green Knight was giant-like with an enormous green beard and an air of handsome civility, wearing sumptuous green and gold clothes and armor. His horse is equally decked in ornate green, and the knight himself holds a branch of holly in one hand and a formidable battle-axe in the other.