If the layer is fully opaque: duplicate your layer (this creates a layer with the same size and position), set the top copy to "Multiply" mode and create a white to black gradient on it. Now, if there are holes in the layer, the "Multiply" on the top layer applies to all layers below. This can be avoided by using a layer group. The groups acts like parentheses in math: the layers are combined as their mode indicates, and then the result is used like a single layer. See attached image for an example (check the Layers list).
Of course in all cases, you can further tweak the gradient using Brightness/Contrast, Levels, or Curves.
Of course in all cases, you can further tweak the gradient using Brightness/Contrast, Levels, or Curves.