shut off the gas at the meter. remove the shut off valve from the iron pipe using 2 wrenches, one on the pipe and one on the valve. install an iron pipe cap using pipe dope on the iron pipe, again using 2 wrenches. Turn on the gas and check the cap with leak detector for any leaks. I am recommending this method as in the future, you may want to go back to gas, or if you sell, the new owner may want gas.
JMHO
Carl
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