Like most things in Photoshop there are probably many ways to accomplish this result. One is to go into Quick Mask mode, use the gradient tool to create a linear gradient across the layer from black to white, return to the normal editing mode and select Edit>Clear from the menu. Another way is to create a Layer Mask for the Layer, and then create a linear gradient as above in the mask. The latter would be my preferred way since you can edit the mask later if you decide more of the layer should be visible.
