It all depends on your situation. Traditionally, GAP insurance will cover damages that are up to $50,000. If you don't have any sort of primary insurance, then GAP will cover the damages. These damages extend to the loss of your vehicle, at the time of the accident. They will also factor in the retail value of the car.
What if I have an outstanding loan left to pay on my car? Yes. They will cover that too. They will only cover up to $50,000. It's best to speak to a GAP representative about it. Every coverage is different. Things vary from state to state. What is covered here in Ohio might not be covered in New York.
If you can afford to still pay off your loans, at the time of your accident, then please do so. Don't rely on GAP covering this completely. Your policy with them might not even include any outstanding loans.