I had gap insurance, got it off t’internet. When my car got written off (not my fault, honest) it certainly helped pay some of difference between car insurance company pay-out and actual cost of car – this is what you need to check with the gap insurer, how much they actually think your car is worth, They don’t pay you upto what you paid for car if it is greater than what they value it at.
But, on the whole, worth it if your car is written off.