How to edit a gradient mask?

The easiest way is just to draw it again. The rate of fade depends on the distance over which you draw the mask. So if you were to draw a mask vertically downwards, it will be 100% opaque all the way from the top to the point where you started drawing. It will reduce in a linear fashion from there to zero at the point where you stop drawing, and be zero all the rest of the way to the bottom. So if you want a very quick transition from 100% to 0%, you draw over a very short distance (perhaps just straddling the horizon). If you want a very gradual transition you draw over a longer distance. And you can edit the gradient mask with the brush and erase tools. So imagine that you draw a gradient mask from some way above the horizon to some way below it, but there is a small hill in the middle of the horizon, so that it is not a straight line. You could first draw your mask and then use the erase brush to remove just the bit that affected the hill.

Ian