Easiest way to do this is to use the Advanced Blending options on a layer placed directly above the image you want to knock-out.

1/ Place you image on a new layer  or double click on the background layer to turn it into a proper layer

2/ Insert a new blank layer above your image  -(you can just click the ‘create a new layer’ icon next to the dustbin at the foot of the layers pallet) and rename it ‘knock out gradient’.  keep the knockout gradient layer selected

3/  Select the gradient tool, if necessary click the linear gradient button and select the default foreground and background colours(to set foreground to black and background to white)

4/ Click the arrow to the right of the gradient display in the options bar to open the gradient picker, then choose the ‘foreground to transparent’ gradient  (by default, this is usually the second gradient in  the gradient pallet)

5/ Click, shift and drag from the bottom of the image to the mid point, to create a gradient to go from black at the bottom of the image to transparent at the top.

6/ In the layers pallet, click Layers style ‘Fx’ option and choose blending options (double clicking the layer also brings up the layer style options)

7/ In the advanced blending panel, drag the ‘fill opacity’ slider to zero percent. Change the knockout drop down from ‘none’ to ‘deep’. (note: ‘deep’ knocks out to the background,  choose ‘shallow’ to knock out just the layer below

8/ click OK to apply the blending options