The three most common causes of banging at the gas water heater are sediment build-up at the bottom of the tank, mis-adjusted burners or overheating due to a faulty thermostat or high limit switch. If the water heater was installed only six months ago, and the problem started just after installation, it is highly unlikely, as you sail, that the problem is sediment.

You may wish to have a representative from PG&E have a look at the burners to ensure that they are adjusted for maximum efficiency. Burners fueled with natural gas should burn with a bright blue flame with a soft blue-green interior and no yellow tips. Finally, open the temperature pressure-relief valve located somewhere near the top of the water heater, permitting some water to escape through the drain line. If steam or boiling water escapes, you definitely are overheating the water and that may be causing the banging. If this condition exists, you should immediately turn off the gas to the unit and have a plumber make the proper repairs. Chances are that your warranty will cover the cost since it's only six months old.