Shadow gradient overlay on sprites

[WoNkY]

I have a number of sprites that rotate. I'd like a gradient mask to simulate lighting like a gradient layer in photoshop.

Thus when the sprite rotates the shadows remain in the same direction.

I have googled and googled to no avail. I'm guessing using a surface but any help welcome.

 

[HayManMarc]

I'm guessing a vertex shader might do it? Not sure, tho.

 

[NeZvers]

Maybe something like this would work.