Opacity gradient layer mask

I hope I can explain this coherently,

What I want to do is fade the opacity of a layer like when you use a black and white gradient on a layer mask...only I want the fade to conform to the edges of a complex selection.

If you had a picture of a donut in photoshop and you made a selection of it, how would you fade the opacity so that the opacity fade went from the outside and inside hole selection boundaries to full visibility in the center of the selection? A radial gradient won't work for me because I need to apply it to other shapes besides donuts. The opacity fade must contour itself to the selection.

The Alien Skin Eye Candy'Gradient Glow' plugin does this. I can make a graytone alpha from a complex selection in there, but I can't figure out how to apply it in Photoshop as a layer mask.

Simply put, I want a layer mask that fades opacity, conforming to my selection boundaries.

I would love to know how to do this, and you'll have my eternal gratitude if you can tell me how.


Edit: Nevermind, I figured it out...sorry for the unnecessary post