GAP insurance is really for those buying the car on finance. If the car is a loss, the money you get back from the insurance might be less than you owe.
But as has been said here, if you need it, you can buy it much cheaper on the internet, there's a lot of profit margin on the dealer prices.
Even if you think you need it, check your normal car insurance. Most policies replace a new car loss with a new car in the first year, and a few even do this for up to two years.