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The NEC prohibits using the Gas Line as Grounding Electrode or Grounding Conductor.
This Inspector needs to take the IAEI's Soares Grounding Seminar; where he will learn the difference between Bonding and Grounding.
The 2008 NEC does require bonding of the metal gas piping.
This can be accomplished by installing a #6 copper ground wire from your electric panel to the gas pipe. Install a ground clamp on the gas line and connect the ground wire to it.
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The NEC is not retroactive; and cannot be enforced in that fashion. According to Code Making Panel 5, the Bonding of the gas line is accomplished by the use of an approved Equipment Gronding Conductor at the on/off switch at the furnace.
If the Inspector insists on having a Bonding Jumper installed, it is sized using Table 250.122, and would be based on the size of the Over Current Protection for the furnace. It is cheaper to install this Jumper than it is to argue with (or educate) the Inspector.