Ok I'll disagree ....
GAP insurance may be useful depending on what you are trying to achieve.
You can get different levels & it depends what risk you are trying to cover.
As an extreme example, If you have taken 100% value 10 year HP finance on a new 4motion with all the options ticked, if your van worth say £90k gets stolen in the first week your finance settlement figure even with an early repayment rebate could be something like £125k - your normal van insurance will not cover that shortfall it would only pay out the £90k. In this case you would want GAP insurance with sufficient value to cover outstanding finance.
Most GAP insurance only covers back to invoice figure & pays out the difference between what you paid & your van insurance payout. If you bought your van cheap & there's been a price increase, that GAP insurance is worthless you could be 20% short of paying for a new van. In this case back to invoice GAP is worthless on a cali.
5 years ago if you were dealing with say a Fiat that lost 60% of its value in the first year back to invoice would have been worth having.
The most important thing is to try and get "new for old" cover for the first few years on your normal car insurance - that way if it is written off & even if you are in negative equity on the finance, you will get a new van & it will just become a substitute vehicle on your finance agreement.
Something else worth noting is that GAP insurance is related to time but you normally pay for it as lump sum payment upfront. Should you sell / replace your vehicle before the end of the GAP insurance period you are entitled to a pro rata refund of your premium.