Goku's victory over Hit in the manga was the result of his perseverance and the ability to surprise Hit with unexpected moves. Also, his experience in battles played a significant role.
In the manga, Gohan never managed to defeat Goku. Their battles often ended with Goku coming out on top or the fight being interrupted for some reason.
In my opinion, it's highly unlikely. Goku has mastered multiple forms and techniques that give him a wide range of powers. Saitama's one-punch style might not be effective against Goku's diverse skills and battle experience.
One great 'Dragon Ball Goku Black Fanfiction' is 'The Dark Ascension of Goku Black'. It delves deep into Goku Black's psyche and his motives for his actions in a really interesting way. The author does a wonderful job of exploring his character beyond what we saw in the original series.
It could be insanely powerful. Goku already has incredible power levels, and Goku Black is no slouch either. When they fuse, it's like combining two nuclear reactors. Their power could be so great that they could effortlessly destroy planets just by flexing their newfound power. They might be able to reach Super Saiyan forms that are far beyond what we've seen in the official series, with power levels that are basically off the scale.
Goku Black in Dragon Ball Legends has a really interesting story. He is an evil version of Goku from an alternate timeline. He shows up with his menacing aura and powerful abilities, causing chaos in the Dragon Ball Legends' universe. He often engages in intense battles against the good guys, and his fighting style, which is similar to Goku's but with a dark twist, makes him a formidable foe.