Sorry to hear of your accident. I do hope you and no other party were hurt.
Gap insurance is available to new cars and can "normally" only be valid for the first three years of its life. In the event of an accident they will give you the cash to purchase a new car of the same spec or they will replace your car with a new one from the factory. Your car was more than three years old so I am guessing that GAP was not applicable to this situation.
The finance company will want all their money. If the payout is less than what you owe then they will come after your for the remainder. They will not be bothered if the settlement from the insurance does not cover the whole amount as the aggrement is with you not them! Before you accept any settlement from the insurance company have a good look at the purchase value of your car and if the offer is below the average sale price then refuse the offer, inform them that you cannot purchase the same car with that amount therfor not a market value offer.
Out of interest, who was at fault? If the other party was to blame then you could sue them for whatever a solicitor suggests!
Wishing you well.....
Just food for thought!