It's never powered by a gas engine, it just has an onboard generator that produces electricity to power the electric engines or charge the battery when the battery gets drained.
It was marketed like that, but in reality there's a planetary gear that allows the engine to directly (mechanically) drive the front wheels. This marketing claimed a pure serial hybrid... up to the day it shipped and GM quietly mentioned that the engine can drive the wheels directly.
At first GM claimed that it happens only above 70 mph, or when the battery was dead. But then GM goes consistently more vague, and them eliminated this completely in the generation 2 revision.
So the current volt is a bog standard parallel hybrid.
