GAP insurance is normaly only available to new cars, a one off payment for cover over a maximum of 3 years. Insurance companies will sometimes offer this on top of your fully comp policy. You can also go back to the car showroom where you bough the car from, they might offer the service too.
I always purchase Gap Insurance but only from the car dealer where I bought the vehicle from. This way I know that in the event of a write-off they will simply order a new car, the same one, or another to the same value. It makes sense to me not to deal with too many different companies.
Just food for thought!