Now that you have your picture you need to add a new layer. So go to Layers>New Raster Layer. A window will now pop up saying Layer Properties click ok for now, you can change the settings later. Now you need to fill in your raster layer with a gradient of your choice. To do this you need to click on your Materials Palette.
click on the arrow on the coloured block. the arrow will open up a small options menu, click the second image (Gradient). You can change the colour of your gradient and the direction its going in.
After you have your colour, fill in the raster layer with it and change your blend mode to colour. You can also change your blend mode to different things like Hue, soft light and overlay. Now go to Layers>Merge>Merge Visible, and your done! :)
My final product:
