here are the steps needed (you are probably familiar with most of them though - I'm just being verbose in case anyone new is interested)
- make any sort of quad mesh (the white_light is indeed the best choice for texture in this case)
- enter edit mode (M)
- in edit mode mode 1 is to assign vertex colors
paint the vertices black on one end of the mesh:
- hold Q and select the color
- select the vertices
- press U to assign the color
(or you can hold E to assign colors one by one)
- exit edit mode (M or Esc)
- enter console (~)
- enter the command: SL
(this automatically sets all the flags and the alpha needed for this)
you can tweak the opacity with the alpha command e.g.: ALPHA 0.5
the default is 0.1
(the order doesn't matter, you can use SL and then set the vertex colors, SL just sets the flags)
also in the editor it will be rendered solid and only shows the additive blending in preview mode