public class CIMGradientFill : CIMFill, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

/// <summary>
/// Create a polygon symbol with a gradient color fill. <br/>
/// ![PolygonSymbolGradientColor](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/polygon-gradient-color.png)
/// </summary>
/// <remarks>
/// 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
/// </remarks>
/// <returns></returns>
public static Task<CIMPolygonSymbol> CreateGradientFillAsync()
{
    return QueuedTask.Run<CIMPolygonSymbol>(() =>
    {
        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
        };

        return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
    });
}