If you have time/patience, Electrolytic Rust Removal is the cheapest/easiest way to remove rust from a gas tank...IMHO.
You can follow this Instructables.com link:
with some slight variation to do a tank. You fill the tank w/ water and the "Super Washing Soda" that you can get at the grocery store. Fab up a rod of some sort to insert into the tank without it touching the sides. Then attach a cheap battery charger to the tank and rod. Follow the instrucables.com link for the proper polarity. The rust will "magically" move from the tank sides (cathode?) to the rod (sacrificial anode?).
I did it to this motorcycle tank that had BAD scale-rust. it took 48hrs to get from 'nasty' to 'slight surface rust'. I could have kept going until i got to clean metal, but it will be a while until this bike will need a gas tank, so i'll finish up then....
Setup w/ cheap Harbor Freight charger and spray can anode support:
Its a pretty neat process. Be sure to use some sort of short-out protection. The rust can build up on the surface of the water and cause a short between the rod and the tank. This didn't happen to me, even with all that gunk floating that you see in the pic, but i guess it could happen w/ enough time. I used a power strip w/ built in breaker to plug the charger into....