An old-time and temporary metal gas tank fix was to put a rubber faucet washer onto a self-tapping, pan-head sheet metal screw. Then, run the screw w/washer into the hole in the gas tank.
I stressed the word "temporary" above, because after a couple of weeks, a common rubber faucet washer would succumb to the effects of gasoline.
(Amazing how quickly a dripping gas tank will create a hole in a black-topped driveway.)
As a more permanent fix on a plastic tank, Snitty nailed it.
Regards,
Gadget