Another difficulat one!
Obviously the best way is to remove and replace the tank.I did see some hypalon tank liners at a Miami show abotu 4 years ago, but the plumbing is still a problem.If the tank is stainless, the rust can't be too bad and gelled gas should be dissolvable with the right solvents, such as acetone or other.Can you get a sample out of the tank to test?If you can, then try to dissolve it in acetone, if it goes up then dilute with fresh gasoline. Then stir around somehow and siphon out and pump through.Be VERY careful about fire!! Then run the engine with several changes of filter.My only real expertise on this is with old cars where the gas tanks can be cleaned and resealed with an epoxy (see Hemmings for ads).Hope this is of some help but you may be best in another forum!ThanksJim W