Why do we need bonding at the gas water heater?
Because your water heater has dielectric unions, so the hot and cold plumbing are isolated from each other.  Bonding the plumbing, attempts to keep the hot and cold plumbing at the same electrical potential. 

If this bond is not in place, it's possible for the hot and cold plumbing to be at different voltage potentials.  Which means if you grab a hot and cold handle, you could complete a circuit.

It also allows the plumbing to act as an effective fault current path. So if wiring faults to the plumbing, a breaker will trip.

A jumper should be installed across any other break in the continuity as well, e.g. water meter, nonmetallic plumbing, etc.