The quick and dirty way is to:
create a new layer
select a rectangle, the size of your "page"
use the gradient tool to create a black to white gradient
set blend mode to multiply
alt drag to duplicate, edit transparency to suitable
The correct way is to do it in illustrator, or any other vector program.
Simply create 2 shapes, filled with a black to transparent gradient. Align the 2 shapes so they butt against each other and have opposing gradients (One at 0° and the other at 180°).
Reduce the opacity of the two shapes. In this sample the right gradient is set to 20% opacity and the left is at 25% opacity. There needs to be some variation in opacity to better convey the "fold" aspect.
Just the shapes on white for clarity....
If you feel the need, based upon the underlying artwork, you may want to set the Blending Mode for these gradient shapes to
You obviously want to create an illusion of surface curvature around the fold. Your case is difficult. The image is quite dark. Darkening it more doesn't look right as the only effect. But you can make the other side brighter - like a reflection highlight.
Sorry for not using Photoshop, but this is possible in any program which has layers, opacity adjustment and blending modes. There are 2 blurred reduced opacity layers with blending mode hard light. One makes the left side brighter and the other makes the right side darker.
The job starts by inserting a new empty layer and filling a rectangular area around the fold with white. The selection should be higher than the page because the next step needs spare height.
Let the new layer have blending mode Hard Light. Remove the selection and apply Gaussian blur. Good amount of blur leaves just the fold line nearly white:
Duplicate the white layer, invert it to black:
Make rectangular selections and delete black and white above and below the print area. Remove black from the left side and remove white from the right side. Reduce the opacities to be able to see the fold properly:
As you see the dark page takes much more effect from white. Balance the sides by adjusting the layer opacities. You can also stretch or squeeze the widths of the blurred zones if needed. It doesn't change their effect depths:
Layer opacities are now 64% for the black and 11% for the white.