Every time I picture a hypothetical 'black Gohan'—like an evil, rage-fueled version of Gohan—my brain spins through scenes from 'Dragon Ball Z' and the way Toriyama teases potential. If we treat 'black Gohan' as Gohan tapping into some dark, unlocked power, there are two obvious ways to compare him to Goku: raw potential versus polished top-tier forms.
Goku has steadily chased and attained godly thresholds: Super Saiyan God, Super Saiyan Blue, and the mind-bending Ultra Instinct. Even if 'black Gohan' were to combine the old-school peak Gohan had at Cell (his Ultimate/Mystic form) with a berserk multiplier, he'd likely outclass most pre-God Goku incarnations—base and early Super Saiyan forms. But against Goku's more recent divine techniques, especially Ultra Instinct's automatic defense and reaction, the gap widens. That said, Gohan's unique edge is his latent power spike when emotionally triggered and his tactical mind; a dark or corrupted awakening could produce unpredictable strengths or new ki-types.
So, in my view, 'black Gohan' can be scarier and maybe stronger than many versions of Goku depending on circumstances, but beating Goku at his absolute best requires either narrative-level power grabs or some divine-level boost. I love the idea because it plays into Gohan's tragedy and potential—definitely gets my hype meter going.