Is it possible to obtain a gradient (aka gradual) distort (or other) effect in paint.net or gimp? How? I mean, for example: 1) pixelate: start on the left with pixels 2x2, and end on the right with pixels 10x10; or 2) motion blur: start from the top with blur distance equal to 2, and end on the bottom with blur distance equal to 20.
Yes, you typically duplicate the layer, apply the weaker setting to a layer and a stronger setting to the other, then add a layer mask to the top layer and fill it with a black-to-white gradient:
After you have created the gradient on the layer mask, you can amend it with Levels or Curves to adjust the transition.
For pixellation, you would use several layers and thresholded layer masks: