How to create a rectangular gradient shadow?

[LaniusSoftware]

I'm looking to create a shadow for a rectangle (either inner or outer) but I can't achieve this with the built-in gradients.
To give you an example, this is an inner shadow:

And this is an outer shadow:

I tried reverse engineering a LinearGradientBrush but it's not very well documented and GradientBrushApplicator is an internal class which I can't access.
Is there some way to achieve this using this library?

[JimBobSquarePants]

No brush is going to allow what you want since the angle of the gradient would have to change as you draw along each face of your rectangle. I would draw a rectangle; blur it then clip out what you don't want.