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](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/PolygonSymbolTwoColors.png)
/// </summary>
{
  //Note: Run withing QueuedTask
  //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
    };
  CIMPolygonSymbol gradientFillTwoColors = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = gradientFillTwoColors.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}

/// <summary>
/// Create a polygon symbol using the ConstructGradientFill method. Constructs a gradient fill using the specified color ramp. <br/>
/// ![PolygonSymbolColorRamp](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/PolygonSymbolColorRamp.png)
/// </summary>
{
  //Note: Run withing QueuedTask
  //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
    };
  CIMPolygonSymbol gradientColorRampSymbol = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = gradientColorRampSymbol.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);

  //Helper method to get a color ramp from the "ArcGIS Colors" style.
  static CIMColorRamp GetColorRamp()
  {
    //Get a ColorRamp
    StyleProjectItem style =
            Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS Colors");
    var colorRampList = style.SearchColorRamps("Heat Map 4 - Semitransparent");

    CIMColorRamp colorRamp = colorRampList[0].ColorRamp;
    return colorRamp;
  }
}