"I'm sorry," he says, gently patting her on the shoulder as he walks out, trying to seem like he's in grief.
It was really immature of her, holding rewards back like that. He had won fair and square, and it wasn't his fault Thales had killed the Cubchoo. She should be thanking him. He had removed a weak tool from her possession. He had helped her. He would return someday to repay her rudeness, but today was not that day.
He exits the Gym without looking back, heading towards the Pokemon Center to get his new 'friend' patched up. He had planned on going straight into the wilds to make him train, but he had done well. He deserved a little reward.