There is an epoxy fix that is especially made for gas tanks.
Go have a look in an auto parts store.
I have used regular JBWeld, to fix leaks in radiators, and gas tanks. And never had a leak. And the 'environment' that it has to stand up to, in a car, is much tougher than that on a bike.
The most important thing about using something like JBWeld is... Preparation of the surface.
If you fix a leak, and it leaks again, chances are you didn't get the surface clean. You didn't abrade the surface, etc.
I once bought a subaru brat, that some teenager, (Brat. LOL), had tried to rebuild the engine on.
Those pancake engines have a bolt in the bottom, that is pretty well hidden. If you don't know that it is there, it is easy to think you have gotten all the bolts out, and the two halves of the engine just need some 'persuasion'.
Well, he used too much persuasion, and broke a big chunk out of the bottom of one half of the engine block.
I cleaned it up very well. Prepared the mating surfaces. And JBWelded the chunk back in.
Almost 20 years later, that engine is still running.
Like I said, if you fixed it with JBWeld, and it didn't stay fixed, chances are, it was a mistake or 'shortcut taken', in the preparation.