Your GAP insurance can expire or no longer be effective after a certain amount of time for a number of reasons. It would depend upon the terms of your gap insurance policy how your specific gap coverage works.
In general, you pay for gap insurance so that you are covered for a specific period of time, namely the term of the loan, and when your loan is paid in full, the gap insurance expires.
With many gap insurers if your car payments go into default, so does the gap insurance, so even if your vehicle does not go into repossession, you may lose your gap insurance if you default on your car loan payments. If you extend the term of your loan without actually refinancing (payment deferment), you may also find you have terminated your gap insurance for the remaining few months of your loan.
Your basic gap insurance coverage "expires" when you either pay off your loan amount or you no longer owe more than your vehicle's actual cash value. So if you owe only $2000 on your vehicle and your vehicle's ACV is $8000, your GAP insurance would normally no longer be active since there is no gap between your loan balance and the ACV for your gap insurance carrier to pay out if your car was declared a total loss.
Gap insurance policy terms can vary depending upon state laws and insurance companies' own guidelines so if you have gap insurance and want to know if there is any type of expiration date for it, read through the terms of your gap insurance policy and speak to an agent from your gap insurance provider.