When did Gohan and Roshi fight?

[SSJ2]

In the 21st, Roshi says that Goku took the Bankoku Bikkuri Sho better than even Son Gohan was able to. So I guess my question is when would Gohan and Roshi have fought? Roshi was already in his prime long ago when Piccolo Daimao terrorized earth, so did he somehow become a fraction of his former power not long after that? Gohan himself far outstripped the level of 21st Roshi/Goku, so I guess they fought early on in their relationship? Why would Roshi need to use such an absurdly powerful technique if they were that weak?

 

[Papasmurf]

I think Gohan trained in the afterlife and grew stronger. He probably wouldn't even be affected by the Bankoku Bikkuri sho at the level he was at in the Baba tournament.

 

[GSM123]

Gohan’s official date of birth is somewhere between 648 and 658, meaning he was born hundreds of years after the Piccolo Wars. Not sure if Roshi had gotten rusty by this point since the Turtle School was still open, but he was definitely an old man when he started training Gohan.