Spark plug gap setting under load

Hello all my name is: Chris
I have a: 1991 gto twin turbo .
Some times under heavy load i.e. pulling away it pops from exhaust .
Could it be bad spark plug gap . so was going to remove clean re gap and re fit . what gap is best.
i have K&N 57i and 3" stright pipe out the back , so not much restrictions.
Could the gap be to big ? someone in one of the shop here said if you make it smaller you can do more boost ?
i have no idea please can some help with correct gap setting.

Thanks
Chris