I need the edge of mask to be fuzzy. The ways i found to do this is either by feather a selection or by stroking a selection with a fuzzy brush. But i need to define the edge transition more specificly. Ideally to create a custom gradient and use it for the edge.
ofn-gradient-along-path script, in combination with a selection from the path to keep only the inner/outer half.
You can also use a plain gradient, and use Levels/Curves to alter it (if it's a selection do this with the "QuickMask".