Gohan is the legendary Saiyaman so it is safe to assume that he can defeat Mercenary Tao. Tao was a threat back in the old Dragon Ball days but since then he has basically gone off the grid. His power level just didn’t evolve along with everybody else’s so he was left in the dust. A quick energy blast would likely finish him off. Gohan wins.
Mercenary Tao is a skilled hand to hand fighter, but he’s not ready to face off against Piccolo. While the Namekian may not have obtained many new power ups as of late, he is still a very formidable foe. He can destroy the whole planet with ease and Tao is just far too slow to stop him. These two fighters are in different leagues. Piccolo wins.
Tao isn’t quite ready to handle someone like Goku quite yet. The Mercenary’s hand to hand skills are actually pretty decent but he can’t blow away an entire solar system the way that Goku can. The difference in their abilities is quite evident and I can’t see any possible way for Tao to last very long in this fight. Goku wins.