Gradient used to mimic neutral density filter along horizon

[gstalnaker]

For anyone looking at this post, linked is an image that shows the problem to solve. A landscape (pond) with background valley left and right. The red line is the "horizon", i.e., the valley slopes, the blue dotted lines the long horizontal that results in extending either side of the valley, and the use of the gradient fill tool (blue to white) that creates a flat horizontal, in this case aligned with the extended dotted blue line that drops to the right. The problem is the area of the large triangle on the right where the lighter part of the gradient begins to wash out the trees.

https://www.dropbox.com/s/fbwnze4jf9pff5...r.png?dl=0

Thanks rich2005. I'll give your plugin a go.

[EDIT] The blue gradient here is simply to show visually (using blue/white for the gradient colors) how use of a gradient is awkward in this situation. In my normal SOP, the gradient is applied on a mask on a layer and it is the typical black/white gradient.