I'm pretty certain GAP insurance is only needed if you finance the bike. Say you finance a 10K bike and the bike is written off. The insurance company pay out say 8K and you are still left owing the finance company 10K + interest.
If the bike isn't financed then I wouldn't worry.
I don't finance either my bike or my car but I have GAP insurance on both.
In 2006 I wrote off a bike that was only a few months old, my policy would have replaced the bike with a brand new one, however, I would not have been able to ride the bike for nearly two years, so I had to take a settlement which was a bit below the price I paid, minus the insurance cover excess. However, the GAP insurance took care of the difference between what was paid out and the invoice price (including the excess) which amounted to a few hundred quid. Had the accident happened a year or more later I would have been a couple of thousand, or more, better off.
This was with BMW's GAP which, in those days, was much more reasonably priced. On this occasion the GAP more than paid for itself and I reckon I'm still in credit despite having had GAP on two bikes and a car since.
Others have mentioned EasyGap, this is who I currently use for both car and bike, they're very reasonable, but I have no experience of pay outs with them. A big plus with EasyGap, and similar, is that if your insurance will replace your bike with a new one if written off in the first year, then you can start the GAP insurance one year later, so you only pay for what you need. Some others, such as BMW's, start when you buy the bike even though your insurance may cover you for a new replacement in the event of a total loss in the first year, therefore you pay for a year you don't need.