Click on "add a mask" at the bottom of the layers pallette. Select the gradient tool (under the paint bucket tool). Make sure that black, white is selected for the gradient in the options bar.
Click and drag in your image to create the gradient. The direction and distance that you drag will determine the characteristics of the gradient .. longer distance = more gradual transition.
If it instead paints your image, then undo and click on the mask icon to make sure you're painting on the mask itself.
Hope this helps.