a gas burner or gas jet for producing illuminating light.
In the film, when the husband goes into the attic at night and lights the gaslights there, it diverts gas from the other lights in the house, so that they dim.
Ingrid Berman, Charles Boyer, Joseph Cotton (I think), and Angela Lansbury (for she got an Oscar nomination).
'Outmoded' means out of fashion
Ingrid Bergman