GAP is an optional product that covers the "gap" between the amount owed on the loan and the vehicle’s value in the event of a total loss. Some exclusions apply, so it is important to review the coverage details to understand the limitations to GAP coverage.
Your vehicle was in an accident and deemed a total loss. Your insurance company determines that the fair market value of the vehicle is $15,000, so that is what they paid out. The remaining loan balance on the vehicle is $17,000. After insurance, you still owe $2,000. If you have GAP, the remaining $2,000 may be covered.