If the dealer can slash the VW price of GAP from £599 to £250 it just goes to show how much commission mark up they're loading onto their supplying insurer's net price. Worryingly, some customers will pay these inflated VW prices.
Personally, I'd always go for new car replacement GAP cover, so if the worst does happen, you can get a new replacement car with the claim money from your motor insurance and GAP insurance at the time of loss, rather than just get what you paid for it.