I don't think we're concerned here with a short in the water heater, or the toilet, etc. Rather, I think we are tryiny to protect from these systems being energised by something else- say, a frayed house wire, or apliance that falles into a sink- by providing a good ground path, so the breaker will trip.
With that in mind, if your pipe is plastic, you need not bond it at all. Any metal pipe needs to be bonded.
The gas line you mention raises an interesting question- what do you do if the gas line is a plastic-coated metal line?
I believe that, in this situation, the bonding provided when the water heater has its' igniter wired up is enough. If, instead, there is a pilot light, I don't really see a need to bond it at all.
