How to create a brightness gradient?

[Ofnuts]

IMHO mostly  a matter of contrast. A rather quick fix;

• Duplicate the layer
  
• On the copy, increase contrast until the left side looks like the right side of the original
  
• Add a layer mask
  
• On the mask, add a gradient white (left) to black (right) gradient
  
• While in the gradient tool, click on the line to add a "stop" and move it around to adjust the opacity change.