Anime-wise, Kaio hyped him up as being at the peak of the Otherworld's power in a manner that would imply he even outclasses Cell. However, he also believed he'd get tired if he were to train Goku, noting that he'd been slacking off. That said, Prime Grand Kaio would be above SS2 CG Gohan, whilst his Anoyoichi Budokai self would rank just below MSSJ Goku.
Manga-wise, it's most likely he'd be inferior to Shin by virtue of Shin being a Kaioshin, so around Gingertown Cell's level at best imo. You could make an argument for Grand Kaio > Shin due to the latter having not been fully trained as a Kaioshin at the time of Boo's rampage, but I'd find that unnecessary when Grand Kaio is completely featless manga-wise. I would bring up the possibility of using his anime self and arguing that Average Kaioshin >> Grand Kaio >>>> Shin, though I personally wouldn't since the Z-Sword scene should place most of the Kaioshin below SSJ Boo Arc Gohan's level.