Unfortunately the ProportionalGradient value for RangeColorMode does not work as expected
when a RadialScale is used. I have logged a public item for this bug report. You can also find your telerik points updated.
As a workaround I can suggest the solution in the attached project which uses a ProportionalBrush mode and two radial scales - one with a range with a solid background color and another with as many gauge ranges as needed. Their backgrounds need to be with close values for background color. Of course the blending effect cannot be achieved in this way.
I hope this is acceptable for your scenario.
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