Mr. Roegels, after trying some method I found the solution for you. Follow the steps below:
1. Draw your mask.
2. Apply Stroke effect.
3. In Effects Control window, Go to Paint Style and choose 'Reveal Original' or "On Transparent'.
4. Now apply Ramp Effect from Generate menu.
5. This will give you a mask with gradient.
Hope this may solve your problem :)
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Use a Shape Layer coupled with Gradient Stroke Shape Operator. Gradient Stroke allows for more than 2-colored gradients and a wee bit more precision with the start/end of the gradient. There is also the additional benefit that Shape Layers are vector imagery which can then be scaled as and when required, without impinging on pixelation issues. You may also combine other Shape Operators to create more interesting effects.
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