Shouldn't this be on "Off Topic"?
@AnimeDefender: Since it's already here it may as well be answered since it'll just be moved anyway.
I believe BoG occurs before the Buu Saga. This would make sense since Goku still had SSJ 3 before the Buu Saga, and Gohan only got his Mystic form during that saga.
Pan was a grown up girl
Well I didn't see her.
The other explanation would be that Gohan can still enter Super Saiyan form, he just never had to in the fight against Buu so he never used it. It was stated that Gohan's power was immense and he wasn't even in super saiyan form, implying that he could enter it if he wanted to.
@eddz99: That was GT. In Battle of Gods, Videl was still pregnant with Pan. However, Gohan does go SSJ during the Saiyans' ritual to have Goku go SSG for whatever reason.
@CerusSerenade: Gohan's Kai upgrade releases all the latent power within his body without the heavy ki consumption that occurs with the SSJ transformations, allowing Gohan full access to his power with minimal strain on his reserves and greatly surpassing what the SSJ transformations could supply him.
@Dream: At this point I'm not looking that far into it. The debate involving Gohan is whether or not he could use SSJ to increase the power of his Mystic form, which he obviously can't. I've stated in the past that Gohan could no longer use his SSJ form, but only as a formality. There's no real proof saying that he couldn't, but he'd have no real reason to, since the Mystic form makes his SSJ form obsolete.
Then there's the problem of non born, non super saiyan, Pan providing enough ki to unlock the SSJG form. Perhaps the God ki has special properties that haven't been explored or revealed. Maybe that's the reason Gohan could use SSJ.