public CIMColorRamp ConstructColorRamp( 
   ColorRampAlgorithm algorithm,
   CIMColor fromColor,
   CIMColor toColor
)

/// <summary>
/// Create a polygon symbol with a gradient color fill. <br/>
/// ![PolygonSymbolGradientColor](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/polygon-gradient-color.png)
/// 1. Create a solid colored stroke with 50% transparency
/// 1. Create a fill using gradient colors red through green
/// 1. Apply both the stroke and fill as a symbol layer array to the new PolygonSymbol
/// </summary>
{
  //Note: Run withing QueuedTask
  var trans = 50.0;//semi transparent
  CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(0, 0, 0, trans), 2.0, SimpleLineStyle.Solid);
  //Mimic cross hatch
  CIMFill solidColorHatch =
         new CIMGradientFill()
         {
           ColorRamp = ColorFactory.Instance.ConstructColorRamp(ColorRampAlgorithm.LinearContinuous,
                                ColorFactory.Instance.RedRGB, ColorFactory.Instance.GreenRGB)
         };
  List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
    {
              outline,
              solidColorHatch
    };

  CIMPolygonSymbol gradientColorFillPolygon = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = gradientColorFillPolygon.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}