I was involved in the creation of Gap insurance about 10 years ago so I can probably shed some light on the product
Basically there are two types:
1. Finance shortfall - in the event of a successful claim the provider will pay out the difference the insurance company payout and the amount still owed to the finance
2. Return to Invoice - in the event of a successful claim the provider will pay out the difference between the insurance company payout and the price you paid for the car originally (less road tax) as stated on the dealer invoice
There are different names bandied around but there are still to this day, only two general types of cover
I would always recommend Return to Invoice as the small extra cost brings much bigger benefits
An oft overlooked benefit of Gap is that you do not have to negotiate with the insurer in terms of payout, the Gap provider does it all
If I was buying Gap I would not look any further than
Gap Insurance by Click4Gap - good prices, great service, no hassle