how do I create a gradient fill in InDesign where both ends have zero opacity to the fill but the middle is 100% opaque? For example a box over a black background has a fill that is transparent to start and then is 100% white in the center and then transparent on the other end.
Create a black/white/black gradient, fill the frame with it and set the Blend Mode of the Fill to Screen. (Black disappears in Screen mode, just as white disappears in Multiply.)
Use a Paper color for the fill and a Directional Feather or Basic Feather.