I used to underwrite it. For the right price, it’s well worth buying. The actual cost of it for the insurer was small at the time, but it did “burn” (i.e., lose money) for the insurer all the time.
For small deposit deals (if I remember this correctly) it’s definitely worth considering as initially your settlement figure may even be higher than the loan amount, plus you lose 20% VAT off the price of the car as soon as it turns a wheel = big “gap”.
An option is to use someone like NFU Mutual or Police Mutual who do 2 year new car replacement. However, you need to check whether they will cover it on a PCP where the ownership MAY pass to you rather than WILL pass to you.
When it works to replace the car, this is (as said above) is RTI or Shortfall Plus cover. This gets much, much more expensive for longer term because of the depreciation.
So, if you can get the above insurers to confirm they’ll cover you in the way you want, I’d insure with them for 2 years from new as long as their price is more or less right. Otherwise just have a gander on the links above and buy GAP from somewhere else.