It depends on which form you think Goku was considering when he wondered whether Gohan could defeat Fat Boo. If he was taking into account a Gohan showing his true capabilities via a rage boost (like before the fight against Dabura) or him at least unlocking (again?) SSJ2 for this fight, then Gohan probably can't even get past Good Boo. If Goku is just considering the power Gohan can access at that moment and isn't taking into account rage boost, SSJ2 shenanigans (which would make things bloated, but it's more likely to be the case imo), then Gohan SSJ defeats even Grey Boo without problems, considering he ought to be still fairly close to fat Boo, unless Grey Boo took 80%+ of Fat Boo's powers or something like this.
Piccolo [Boo saga] vs Future Trunks SSJ [killing future Imperfect Cell]