Open any image.
Make new layer by clicking Create a new layer icon ().

Set foreground color to red and background color to white.
Select Gradient Tool () and apply to the layer you have just created. Hold Shift and apply from right to left.
Make a copy of the background image by dragging it to new layer icon (). Position it on top as shown to the left.
And finally set the Opacity of the layer to 50%.
(You can do this effect by setting the blending mode to Overlay too, see which one suits you)

And finally here is your easy smoothly applied Gradient effect.