Someone driving a Gen 2 Volt with plenty of power in the battery and wanting to run out of gas is either experiencing an FMM episode (when the engine runs and you can’t use the battery until you do something about the "old" gas) or is trying to empty the tank by using Hold Mode.
Sounds like you’re using Hold Mode to use up your remaining existing gas, and want to use it all up before you add even a small amount more in order to maximize the delay until the next FMM might occur.
Note that the gas range estimate, when it switches from a number to Low Fuel (with ~1.5 gallons of gas remaining), can be misleading. If you haven’t used gas much recently, the estimate is likely a low number, reflecting "cold engine" gas mileage. Once you choose to run the engine until the gas is gone, the distance you may drive on the remaining gas may significantly exceed the "cold engine" gas range shown just before Low Fuel appeared.
Yes, when you run out of gas, you’ll experience a Propulsion Power Reduced episode until you get more gas, but will be able to use all available battery power. If the driving conditions at that moment require only a moderate power demand (e.g., you are cruising smoothly on the freeway), it is possible the primary issue you will encounter is less responsive acceleration.
Be aware that if you run out of gas (my preferred method of ending an FMM, and I’ve experienced 3 of them in 6+ years in my 2012 Volt), once you do add gas, the engine performs a self test to confirm all is in working order before the car returns to normal operations. The Gen 2 manual has this to say regarding Out of Fuel/Engine Unavailable:
"If the vehicle runs out of fuel, or the engine will not start due to a malfunction, the vehicle can continue to be driven in Electric Mode until the current charge is depleted. The vehicle will have less responsive acceleration. DIC messages indicate reduced propulsion power, that the engine is not available, and the need for fuel or service. Once the vehicle is refueled, or the malfunction is corrected, the engine will start the next time the vehicle is turned on to perform a self test, and DIC messages will not be displayed. Once the engine starts successfully, normal operation will continue in either Electric or Extended Range Mode. The engine may stop running after the self test is completed, based on the current mode of operation."