Yes, typically GAP insurance is available and can work for a used car just as with a new car if the vehicle is financed in a way in which you owe more than the car's current value and thus have negative equity in the vehicle.
State laws and insurance companies’ guidelines vary but there are GAP policies that are available to be placed on used cars that are financed. GAP insurance is beneficial when the value of a vehicle, whether new or used, depreciates while you still own money on the loan or lease. When you owe more on your vehicle than it is worth this is termed being up-side on the loan and GAP insurance can help you if your car is declared a total loss during this time.
GAP coverage provides for the difference between the actual cash value (ACV) amount paid out by your primary insurer if your vehicle is a total loss or stolen and the amount you still owe on the loan or lease. Some insurance providers might have restrictions on their GAP insurance policies but it should be possible to obtain GAP insurance for a used car was bought using financing with a lien holder.
When you purchase an auto insurance policy online with CarInsurance.com, most of our carriers offer gap coverage. It is labeled Loan/Lease Gap coverage. You can purchase it easily with your policy for very little premium. If you don't have a CarInsurance.com policy on your used car than CarInsurance.com offers GAP Insurance as a separate policy through this online partner.