I do not see the connection between the gas cap and an O2 sensor.
If you were told to replace the gas cap, by a dealer or mechanic who scanned the code, chances are you have an EVAP system leak.
Meaning gas fumes are leaking into the atmosphere, when its designed to be circulated into the intake and than burned off.
One of the many parts of the system has failed or leaking. A hose, connection, the evap canister, or the vacuum system that is used to detect leaks has failed...
Not tightening the gas cap, or a gas cap that has broke somehow can also throw the code.
