Okay, first set the brush that you want faded to a func_illusionary. This makes it visible, but not solid. Set the contents box to either empty or light, (doesnt really matter) then make sure the black bit of the fade texture is where the light would start to fade. I.E., its at the end away from the window or whatever the source of light is. In other worsd, the pure black is where it will appear invisible, as additive brushes render black transparent, sort of how like solid renders blue invisible. Change the rendermode to "Additive" and the renderamt around 40 or so, experiment with different values to get a different effect.
-Rabid