The web site below sells Gilson belts - you need the part number. If you do not have the part number, maybe if you contact them, they can match the belt to your tiller number.
Also, another web store, Jacks small Engines (jackssmallengines.com) states that they carry parts for Gilson, but I did not find Gilson in their directory. You may have to contact them.
One of the web sites mentioned that at some point Gilson was bought by Lawn Boy. If you go to a local Lawn Boy dealer, they may be able to match up a belt.
I have a 1978 WheelHorse tractor. The drive belt for the mowing deck broke. I went to Home Depot, spend a good 1/2 hour measuring belts, and found one that works perfectly. If you go to your local outdoor equipment shop, especially one that sells, repairs tillers, they may be able to match up a belt for you. A belt is a belt is a belt. If you go to a "good old fashioned shop" that will spend the time, they can measure the old belt and hopefully come up with a match.