Bonding of the different systems in a building are done close to the electrical service. A gas appliance that utilizes electrical is a crossing point for two of the systems that are required to be bonded to the service. I have seen techs bond the furnace to the gas pipe but I really think it a problem for two reasons. If one of the systems does become accidentally energized there will be a parallel path to the electrical grounding system. When the system is not energized the bonding jumper from the electrical bond in the furnace to the gas piping system will provide a path for circulating currents between the two systems... these currents can cause problems in the long term. In the event of an energized gas piping system the current will flow to the system with the least impedance which more than likely will be the electrical system... thereby, sending the current from the gas pipe to the electrical system at the connection point in the furnace which will have less amperage capacity than the main bonding jumper that will be sized for the size of the service entrance cable. Odds are the ground/bonding wire that is sized for the breaker that is feeding the furnace will be damaged and could go undetected causing safety issues and other problems at a later date. I have done services in commercial buildings where the gas comes in one end of the building and the electrical service comes in the other end of the building... with a 600 amp service it gets expensive running a 1/0 bare copper wire the length of a 300 foot building just so you can get from the gas meter to the electrical service.