Using a DynamicResource to set the GaugeRange Color property does not work:

<!-- does not work -->
<gauges:GaugeRange Color="{DynamicResource GrayBackgroundColor}" ... />

Using a StaticResource works as expected:

<!-- works -->
<gauges:GaugeRange Color="{StaticResource GrayBackgroundColor}" ... />

Side-Note:
Using DynamicResource for Indicator Fill works as expected.