Probably not gauge if it has ability to show empty and full. And if I am wrong only repair would be to replace. Most likely it will be sending unit in gas tank. If you look at tank under deck you will see a round piece with fuel lines and wires going into tank. That is not only one of the fuel pumps but sending unit as well....good luck
It may be a bad sender unit. I replaced the stock sender in my boat with a reed type from WEMA USA. It is not the traditional float/lever type but rather one where the floating mechanism rides up and down on a stainless steel shaft. It is very accurate with no oscillations even when the boat is rocking like a sea-saw. No modifications to the existing tank are required so installation is a cinch. Worth every penny. It could also be a bad wire connection which can be checked for continuity between the gauge and the sender unit.