Do I need to shut off the gas to my water heater, when I shut off the cold water intake to fix a leaking hot water faucet in my basement sink?
It's not a bad idea to turn off the gas while you work on your water system as the water level inside the tank may become unacceptably low, or pressures to high if you shut all the valves off; triggering the T&amp;P relief valve.

You must turn off the gas or power if you have to drain the entire system and the tank.