In the US at least, metal water piping is required to be bonded using a jumper that is sized relative to the service, even when the underground portion of the pipe is _not_ suitable as a grounding electrode.

_Other_ piping systems are required to be bonded based upon the size of the circuit which might energize the piping. The furnace circuit, for example, or the gas range circuit, with bonding done from the egc of those circuits to the pipe.

Most of the gas hot water heaters that I am aware of don't require electricity at all; though I have seen modern heaters with forced draft systems that require electricity for the blower. If there is no circuit to the heater, and the water pipes to and from the heater are plastic, then bonding the heater to the steel gas piping probably does nothing to help ground the gas pipe...though if the pipe is grounded, then it might help ground the water heater [Linked Image]

-Jon