CEL light comes on when I travel and use different gas?

[Donal]

I hope you arent trying to say the PCM monitors the fuel pressure????

No, I said fuel tank pressure not fuel pressure. Please read for clarity and context.
Clarification to other readers.
The fuel tank pressure sensor is mounted in the fuel pump mounting flange and is a component of the evap system. If a component if the system fail to function, the tank pressure may be sensing a positive tank pressure that exceeds expectation.

The op has combined two different situations, from two different times, in their post. The misfire codes are old and the new CEL has not been scanned.

 

[bmidd]

My P0300 code went away after the fuel tank and fuel pump were replaced. The top flange of the tank was misshapen and the pump wouldn't seal. You could never put over 3/4 tank or it would leak.

 

[boba1111]

Update:
Ok so its been a week and I have driven some distance. The CEL is off and remains off and since I do not have my code reader in PA there is not much more I can do.
And the wife and i decided we need a newer vehicle. We did LOTS of research and settled on Hyundai Palisades Limited!
We pick it up tomorrow and my mechanic has a buyer for the Denali so ALL is good!