There are two primary safety tips for removing an old gas water heater, according to DIY Network and Do it Yourself; disconnecting the gas flow and removing the hot water. Without doing both prior to removing the old heater, the removal process can cause fires or burns.
Turning off the gas should be the first safety step in removing the old gas water heater. Keep a fire extinguisher within reach while shutting down the gas. Turn off the gas supply by using an adjustable wrench to close the gas valve. After closing the valve, examine the pilot light, which should go out without any gas providing fuel for the flame. Sniff around the area for the smell of gas as well to determine if the supply is completely shut off.
After turning off the gas, remove the hot water from the heater to avoid being splashed and burned while removing the water heater. Removing the water also lightens the heater enough that it can be more easily moved. Close the cold water valve to prevent more water from entering the heater while draining it of hot water.
To remove the hot water, turn on the nearest hot water faucet and let it run until all of the water is removed from the system. The system is emptied once the water stops flowing from the faucet. Hot water can also be removed by draining it directly from the heater using the out valve on the heater. Make sure there is a nearby floor drain before opening the out valve to allow the water to escape.