public CIMFill ConstructGradientFill( 
   CIMColorRamp colorRamp,
   GradientFillMethod gradientFillMethod,
   int interval
)

/// <summary>
/// Create a polygon symbol using the ConstructGradientFill method. Constructs a gradient fill between two colors passed to the method. <br/>
/// ![PolygonSymbolTwoColors](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/PolygonSymbolTwoColors.png)
/// </summary>
/// <returns></returns>
public static Task<CIMPolygonSymbol> CreateGradientTwoColorsPolygonAsync()
{
    return QueuedTask.Run<CIMPolygonSymbol>(() =>
    {
        //gradient fill between 2 colors
        var gradientFill = SymbolFactory.Instance.ConstructGradientFill(CIMColor.CreateRGBColor(235, 64, 52), CIMColor.NoColor(), GradientFillMethod.Linear);
        List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
        {
            gradientFill
        };
        return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
    });

}

/// <summary>
/// Create a polygon symbol using the ConstructGradientFill method. Constructs a gradient fill using the specified color ramp. <br/>
/// ![PolygonSymbolColorRamp](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/PolygonSymbolColorRamp.png)
/// </summary>
/// <returns></returns>
public static Task<CIMPolygonSymbol> CreateGradientColorRampPolygonAsync()
{
    return QueuedTask.Run<CIMPolygonSymbol>(() =>
    {
        //outine
        CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(49, 49, 49), 2.0, SimpleLineStyle.Solid);

        //gradient fill using a color ramp
        var gradientFill = SymbolFactory.Instance.ConstructGradientFill(GetColorRamp(), GradientFillMethod.Linear);

        List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>
        {
            outline,
            gradientFill
        };
        return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
    });

}