I speak as someone who has worked in a car dealer for the last ten tears. Not all insurance products are a con to make profit. Like any insurance, it is a risk transfer mechanism. What Guaranteed Asset Protection (GAP for short) is designed to do is make up any shortfall from an insurance policy in a total loss situation (i.e. a write off). Understand that most insurance policies cover trade value at time of claim. As time goes on, the value of the policy will grow, you are effectively covering the depreciation on the car.
Car dealers can/do offer two types of policy.
One will cover the full amount against purchase value.
The second will make any shortfall against a finance agreement.(lesser value), on dealer sold finance.
GAP insurance will cover you any way you decide to pay, either cash or finance/loan.
You may find your current insurer will give you this for your first year, GAP is likely to run for 3 years.
You MUST have fully comprehensive insurance if you wish to take out the policy or wish to finance a car.
Hope this answers your question.