GAP insurance is an insurance for people who have finance on a car. Example, brand new car costs you 25k, you pay for it monthly, in 18 months time you write it off, at this point it's only worth 18k. Insurance only give you 18k to replace your car but you still have outstanding finance of say 22k. So, you have to buy a car worth 18k that you actually owe 22k on. Yes, the car you owe 22k on is now only worth 18k, but for the sake of £200 or so over three years, you may as well get a new car if the worse happens, or a car that's worth what you owe on finance.
As with most insurance, it's a gamble, if you don't need it it's a waste of time, but if you do, it isn't.