If your house is plumbed with copper or galvanized pipe you should have a bond to the cold water pipe already. A shower valve completes the bond to the hot water side.
The gas line will be sufficiently bonded by the gas appliance's electrical connection. This assumes you have an equipment ground on the circuit that provides power to the appliance.
When was this house built? Is the water line to house metal? What kind of gas piping do you have?