Few coatings are good for sealing gasoline. One is POR15 which will recoat the inside.
Such a tank sealer in use you would tape it up, pour in the gas tank sealer, rotate it around etc.. pour out the excess.
You would have to clean the tank good, like put in gravel, water, soap and shake it a lot.
Otherwise, you can use maybe JB Weld epoxy, or some other gas resistant epoxy, but it likely is only temporary fix.