It is true that gas filled bulbs (mercury or sodium) or tube lights emit more light compared to ordinary filament bulbs for the same power input. Basically when an electric current (flow of electrons) passes through any ordinary material both heat and light energy are developed in the body. In filament bulbs the current passes through a thin wire made of tungsten.
Because of its material structure more light is generated than heat. However a part of the energy (power) is dissipated as heat in the body. Also the light emitted is limited by the surface of emission and the length of the tungsten wire.
In the case of bulbs filled with mercury vapour or sodium vapour the electrons move from cathode to anode through the vapour. These gas molecules are good conductors of electrons and the electrons passing through these molecules emit light. Comparatively lesser power is dissipated as heat. Also the electrons passing through those freely suspended molecules in three dimensions have greater volume and space compared to the electrodes through the filament, leading to higher intensity of light.