Now select gradient tool from the left panel. Make sure that your foreground color is “black”. Now from the option bar change the mode to “foreground to transparent” and type to “linear gradient”. Change the blend mode to overlay. I have written a whole tutorial on Blending Modes in Photoshop and you should check that tutorial out if you are not aware of the power of Blending Modes.
Now apply the gradient as shown below. If the result is not satisfactory then apply one more time.
Your photo should look like this.