Does that work for you?
Check in these links and product help
Try this experiement. Create a new file with a transparent layer. Duplicate the layer and create a shape using the marque tool. fill the shape with black. Select the gradient tool. Go to the edit button and set up the gradient to go from opaque to transparent. With the shape still selected apply the gradient tool.
the result I get is no change to the shape, just stays solid black.
Ah, I think I understand.
This gradient function adds pixels, or not in the case of transparency, as opposed to removing them. So adding a gradient that goes from black to transparent on a black surface will do nothing.