Actually, you are kind of as enlightened as him. The 1944 movie you are talking about was a remake of a 1940 British film Gas Light. But that wasn't the source of the term either...
The British film was based on a 1938 play of the same name. The gas light come from it taking place in 1880s Victorian London, before it had electrical lights.
The British film Gas Light is very good and well worth watching.