See NFPA 70 Sec. 250-104(b) and NFPA 54 [Sec. 3.14(a)].
NEC 250-104(b) requires you to bond the aboveground portion of a metal gas piping system to a grounding electrode system for safety reasons. NFPA 54; Sec. 3.15 does not allow you to use aboveground portions of a metal gas piping system or its components as a conductor in electrical circuits.
The difference between Grounding and Bonding leads to a lot of confusion.
The gas pipe needs to be Bonded to the Service Ground so that it maintains the same potential as the electrical system. This is done at a single point. If the gas pipe is used as part of the grounding path there is the risk that current could flow along a pipe that contains a flammable gas.
Saying that the gas line needs to be Grounded is a common holdover from when the differences between Bonding and Grounding were not clearly defined.
