Gas company has a shut-off somewhere in the line. How could they shut-off your supply if you didn't pay the bill?
I am not a plumber......but I would install the new valve without shutting off the gas. Just ventilate the room real well and make sure all sources of ignition are turned off such as a water heater pilot.
I agree with Alan, do not try to change this valve on the fly it's not water or air it's BOOM gas. How many people have try-ed to install valves on the fly and something goes wrong and it's open with no way to shut it off and panic sets in? I have.
that old gas shut off is no longer code for the very reason you are having a problem with it. change it to a new valve. If you want to discount the knowledge of the people who do this type of work every day then do it your way.
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