If it were just that, Gabby might have not reached the same conclusion but when he saw how he used micro movements to beat down on Macbeth despite his sword skills, Gabby became sure that there wouldn't be a stronger opponent than him among the participants.
Gabby felt that he had to remedy the situation somehow or they would end up on the losing side despite how far they had come. It was even possible that they might be unable to escape and might have to hand over their cards to get his forgiveness if things continued to escalate with him. He knew Ibeniah was too vengeful while Macbeth was too proud to do that. Therefore, he being the leader had to lower himself in front of Zavier.
