Ah, that’s something else that we were not aware of. In that case, my suggestion for processing the pulses in the emonPi’s front end can never work, because you never see the actual pulse from the meter.
I have never needed to find out in detail how the interfacers work. Paul created emonHub but even though others have changed things since, he is still probably the best person to tell you what might work and what might not. As I understand it now, in essence your problem is the interfacers as they exist process and send the data onwards when a message arrives. What you seem to need is an interfacer that picks out the gas data, notices a change in the count, and only then sends the data onwards, probably under a new NodeID.
My personal view is don’t split the discussion into two threads. If you do, the second will only be the solution to the first, and that will confuse everybody. If it becomes sensible and feasible to split the “new interfacer” discussion out, that can be done later.