You may think that the electrician neglected to bond the gas line, but it gets bonded by hooking up the furnace or range properly.

NEC 250.104

The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means.

"GROUNDING" is NOT the requirement for gas pipe. It must be BONDED to the one and only GROUNDING ELECTRODE SYSTEM.

If two metallic systems such as piping, wiring, coaxial cable, or metal ducts have greatly different electric potentials, an arc can form between them.

The ground can have a different electrical potential from one end of your house to the other. That is why a swimming pool has a grounding system that goes all the way around it. Lightning can also cause the potential difference.

When one ground rod is attached to the center tap of a transformer, and the other is not, another opportunity for a dangerous situation is when an open neutral occurs, which is not all that infrequent.

If the gas line coming out of the ground is metal, then it is already grounded. This is what creates the potential difference in the first place that makes it necessary to bond the gas pipe to the electrical system. (In this case, the National Fuel Gas Code and #6 AWG wire should be used) If the metal pipe is not connected to ground, it doesn't have a potential difference. If it is only running through a crawl space, and insulated from ground by the wood framing members, and a plastic pipe from the utility company, it poses little to no real threat in general until a low impedance path to ground is established. If the metal pipe goes into the attic, it may serve as a lightning rod. In that case, it should (not required) have a direct path to ground as soon as it gets down to ground level in addition to a proper bond to the electrical grounding system.

Here is a fairly simple explanation of the differences in codes.

http://www.csstfacts.org/protecting-gas-piping.aspx