The Gap insurance is to cover the car should it be totalled and you owe more than what the car is worth and the auto insurance does not cover it, the gap covers the difference. http://cars.about.com/od/buyingadvice/a/gap_insurance.htm
The Gap insurance is usually sold to you when you take a loan and is good for the life of the loan. You should only have to pay the amount which is included in the loan payment.
If the vehicle is paid off early there is usually some conditions where if there is money left, you can cancel the Gap insurance and get a refund. This is some information on Gap insurance: http://www.ehow.com/info_8357466_happens-gap-insurance-refinance.html
I hope this information is helpful.