You can't get rid of the rust, but you can add to the tank's longevity. Inspect the bottom of the tank to be sure there is no rust starting to come through. If the bottom is OK, the tank is still OK.
Remove the gauge assembly from the top of the tank. Drain all but about a gallon of gas. Put a couple of handfulls of nuts and bolts into the tank. Cover the port with duct tape and then install the gas cap. Shake rattle and roll the tank to free any loose rust. Then completely empty the tank.
If there is sludge in addition to rust, drain the tank and use high pressure water from a garden hose to flush out the tank. Then add a gallon of water and do the nuts and bolts thing. In this case, when finished add a bottle of rubbing alcohol to get rid of any water remaining in the tank. Then drain that.
Before replacing the gauge unit with a new gasket, replace the cork float with a neoprene rubber stopper ("cork") of about the same size.
In the future use E10 gas and it probably won't rust any more. Also, keep the metal tank as they aren't making them any more and they are far superior to the plastic ones they sell today.
