This time we'll do it a bit differently. We'll use the brush tool with very soft settings (see picture). Create a new channel, use the selection from the previous step, use a white brush and click it until you have a gradation you like.

But, it will be going the wrong direction. So, you'll invert it will it's still selected (ctrl-i). Then you may need to do a Select>Modify>Contract 1 pixel, invert selection and delete to get rid of any fringe artifacts.

Your final channel should look like the one in this picture.