@MapleTrees Oh? And what did that imply? I'm really curious because that's admittedly a very confusing line, and different translators even give it completely different meanings (I'm Italian, and in the official Italian version Gohan is actually thinking "I didn't think (he/it) was that strong..." there).
@Labba it implied that Gohan was far stronger than Goku and didn't think Goku was actually using his 50% power here. This was because he was comparing Goku's power to his own. Since Goku's 50% power was lower than his 50%, he assumed that Goku was still hiding some of his power because Gohan has always viewed Goku to be someone stronger and the Earths savior.