I will be away for 2 months and want to know whether or not I should shut down and drain my gas water heater, or simply turn the temperature down. Someone will be checking the house every few days and could run the hot water to prevent sediment etc.
Air in the tank could be worse than leaving it full. Turn it down or off but have the water run periodically. You will want to do that anyway to keep the drain traps full of water.