Open up your picture in Photoshop and go to Layer >> New >> Layer to add a new layer.
Now click the gradient tool on your side bar. There should be a new tool bar on near the top of the screen that looks like this.
Now click the little arrow as shown in the example and pick whatever gradients you have.
Now next to that is this:
You can choose which ever to use on your picture. If you hover over them, it tells you what they are. They are as follows: Linear Gradient, Radial Gradient, Angle Gradient, Reflected Gradient, and Diamond Gradient. I will be using the Linear Gradient. You can choose the other settings on the gradient tool bar as well if you want.
Now with your mouse click on the upper left hand corner outside of the picture. Now you don’t have to start where I do but this is how I’m doing it. You can start your gradient anywhere.
Now drag your mouse across the picture while still clicking to the bottom right hand corner. When you release your mouse a gradient should show up on your picture.
Now click F7 so that your Layer Palette shows up and change the top layer to whatever Blend Mode you want. I set mine to Screen.