If it actually is the fuel cap leaking, get a new one. Chances are (more realistically) you have an EVAP system leak if your gas cap was seated properly to begin with. Any large leak in the system that is similar to the gas cap being off will trigger a "gross leak in the evap system" code.
Even if you tightened your gas cap it may take a while for the light to go off. Disconnecting your batter will probably cause the light to go off, and if there is still a problem the light will come back on. If not, you're good to go.