Assuming that your current insurance is comprehensive, there is no reason that the GAP insurance should dig deeper to see that the car has been continuously insured during your ownership � they will only be interested in the fact that you have current insurance cover.
My understanding of GAP insurance is that it covers the gap between the outstanding finance amount and the value of the car. It may be that no GAP insurance is payable, because there is no difference between what you owe on finance and what your insurance company offers you. In any event, it should not be a massive amount.