For one, he's green. Everything he is wearing is green. His beard, his hair, his armor, and even his horse are all green. This is not very natural, and it is certain to get you noticed.
He also stands at least a foot taller than any of Arthur's knights. He is well-proportioned, and all the knights stare at him in fear as he walks in holding a holly branch in one hand and a battle axe in the other.
It is at this moment that he explains he has come in peace. He explains his challenge, and then allows Gawain to behead him. If the two physical traits weren't enough to intimidate everyone there, the fact that he pops up, tucks his head under his arm, and rides off to wait for Gawain to find him within the next year is enough to get people talking.
He is green and kinda buff, he has many intimidating looks about him especially how he gets up after he gets beheaded; I dont know if the story gave any more physical traits but if you want to know more I suggest you read the story it is a real good theme based story :)