"Oi, you stay away from my butt" He is dying, he is laughing so hard that if Gokudera really wanted to he could probably get at least relatively close to lighting his butt on fire before Yamamoto ran away. He will cover it with his hands for good measure though, just in case or really more to laugh harder at this silliness.

"I'll steal the hat then" He grins, still cute and besides Gokudera should know, hat or no hat if Yamamoto wants to pet the ears, Yamamoto will find a way to pet the ears.