You’re not actually "driving on gas." The Gen 1's "gas mileage" is the distance you travel during which one gallon of gas is being used by the engine to run the generator.
I’ve been driving my 2012 Volt for 6+ years. My understanding is that when a Gen 1 Volt "seamlessly transitions to gas," what really happens, in effect, is that the large traction motor is "unplugged" from the battery and "plugged" into the generator’s output, and then runs on gas-generated electricity ("electric" miles = "grid electricity" miles, "gas" miles = "gas-generated electricity" miles). Under certain lower demand operating conditions (think of conditions experienced when cruising smoothly at > 35+ mph), the generator motor is clutched to the drivetrain to increase overall efficiency (better generator fuel consumption rate efficiency = better gas mileage). In this split-power configuration, ICE torque contributes directly to the propulsion torque.
A High Voltage Charging Error message suggests there’s a problem getting the high voltage battery to charge properly. Is it a faulty battery or a charging system problem? You could try using Mountain Mode’s "recharging" feature. By switching to MM while driving or when parked (the self-charging chevy Volt video shows a 2012 Volt being MM recharged while parked), a fully depleted battery should be recharged to the ~4 bar level in ~15 minutes using ~0.36 gallons of gas.
Switching to MM in a 2011/2012 Volt before the ev range has dropped to 0 will allow you to switch back to Normal and regain use of that MM-recharged power immediately. Using MM after the battery is fully depleted may require you to turn the car off and back on again to gain use of that MM-recharged power. When you use MM-recharged power in your 2012 Volt, it will count as Electric Miles without changing the kWh Used number (it’s not grid power).
If MM can successfully recharge your battery, then your error message would seem to indicate a problem with the recharging-from-the-wall system, perhaps a problem with the plug on the side of the car or the recharging circuits under the hood.