what O2 sensor was it sensor 1 or sensor 2 ?

if its sensor 2 ( post cat ) then you are correct, its because the cats been removed, although there are kits to trick the ECU to give a false signal back to the ECU so it assumes the CAT is working fine.

if its sensor 1 then there maybe a problem.

as for the fuel gauge code, the engine ECU will use the fuel level sensor as part of the misfire detection processes, if its not a present code and the gauge works fine, clear it and see if it returns. this will have no effect on the way the car drives as its purely for a diagnostic aid for the ECU.