80-85%. The statements don’t make it sound like he was dramatically weaker, but that combined with his lack of training resulted in him being a far worse fighter.
Chapter: 452 (DBZ 258), P1.1-4
Goku: “Gohan, it’s your turn next, but have you trained properly?”
Vegeta: “Unfortunately, it seems he got carried away in peace and didn’t do any significant training. Our powers are higher than his now…Though I suppose there’s no telling what would happen if he snapped and went into a frenzy… ”
Kaioshin: “S-so that’s why these 3 have such composure...In a pinch, they can put forth tremendous power, like Son Goku displayed momentarily…”
Chapter: 455 (DBZ 261), P6.1-2
Context: as Dabra fights Gohan
Goku: “Magic, huh? [Dabra]’s way stronger than I thought, ain’t he?”
Vegeta: “Hmph…Even so, he’s not an opponent [he? we?] can’t win against. [Gohan]’s so pathetic…So much so that he was stronger as a brat…”
Goku: “He really did slack off!”