A gas pipe must be bonded to the electrical service, but it is not permitted to use it as a ground conductor.
You need to run a legal grounding conductor (sized per the relevant table) back to the Service point.
This has been the case for many years; it sounds like this is just the first time that an AHJ noticed and red tagged it.
Can you reference a code article that clarifies that? Also as far as an AHJ noticing it, it was recommended by one of the city inspectors about 8 years ago and we point it out on every inspection with no objections until yesterday.