Why on earth would you want GAP insurance? In my opinion it should be the subject of the next mis-selling scandal. Your ordinary insurance will replace your car to the same value it is worth at the time it is written off so you should only lose the excess on your insurance policy. Presumably you will be saving for your next new car so you can use the money saved so far to top up what the insurance company give you to buy a new car if you really want one. If you write off your car when it is one year old your insurance should allow you to buy a replacement one year old car. You have lost nothing. If you like to line the pockets of the insurance companies the go ahead and buy GAP insurance.
Years back I would agree, my then GF had a 2001 Clio it was a 18 months old when she was making a right hand turn and the oncoming car hit the side, the car was written off, the amount from the insurance was obviously a shortfall, but the GAP insurance made up the difference so that she could again buy a 6 month old car.