I have never purchased GAP insurance....
I am very skeptical and would really do some research on it first.
However, in the 2 mins of looking I did already find the following about GAP insurance!
http://www.carbuyingtips.com/insurance.htm
Don't fall into the gap!
If you owe more on your car than it is worth, if you lease, or if you put down less than 20%, you should get Gap Coverage from Gapinsurancequotes.com. Most people refer to it commonly as "gap insurance". Don't even bother buying it from dealers, they charge $500-$700 when you can get it online directly from Gapinsurancequotes.com for less than 1/2 the car dealers. If you owe $20,000 on your car, but it's only worth $16,000, you're upside down. You total the car, or it's stolen, your insurance company gives you $16,000. You must still come up with $4000 to pay off the bank, plus your $500 deductible! Gap coverage protects you against this. The better ones cover up to $500 of your deductible.