"Gas" mileage is a debatable term for a Gen 1 Volt.
MPG (when written in all caps) = (total electric miles + total gas miles) / total gas consumption
This is a meaningless number for a car capable of moving down the road using no gas at all. The higher the number, the more time you drive using grid power from the battery and the less time you drive using the engine.
The window sticker gas mileage is the expected distance you can drive when you are driving with the engine running (MPGcs, or mpg in charge sustaining mode, i.e., Extended Range Mode).
Keep in mind a Gen 1 Volt is propelled by electricity 100% of the time. In effect, when you transition from Electric Mode to Extended Range Mode driving (when the battery is depleted or when you switch to Hold Mode), the motor is unplugged from the battery and plugged into the generator output. GM refers to this as "electric like" driving.
Whenever the Gen 1 Volt is in single motor configuration for Extended Range Mode driving (e.g., when in stop and go driving, when accelerating quickly), the Gen 1 Volt is an all-electric car running entirely on gas-generated electricity. When cruising down the road (>35+ mpg), the generator motor may be clutched to the drivetrain (split-power configuration), which smooths out the "off/on" engine cycles and allows the engine torque to flow through the generator to the wheels. This increases overall efficiency, reducing the generator’s fuel consumption rate, and thus improving the "gas" mileage.
One could say the Gen 1's "gas mileage" is the distance you can travel on the amount of gas-generated electricity the motor pulls from the generator’s output during the time it takes the generator to burn one gallon of gas.