Вы можете создавать составные градиенты, используя кисть для рисования. Например, вот градиент алмаза, который вы можете вставить в окно для тестирования:
<Window.Background>
<DrawingBrush>
<DrawingBrush.Drawing>
<DrawingGroup>
<DrawingGroup.Children>
<GeometryDrawing>
<GeometryDrawing.Geometry>
<RectangleGeometry Rect="0,0,100,100"/>
</GeometryDrawing.Geometry>
<GeometryDrawing.Brush>
<LinearGradientBrush StartPoint="0,0" EndPoint="1,1">
<GradientStop Offset="0.0" Color="Black" />
<GradientStop Offset="1.0" Color="White" />
</LinearGradientBrush>
</GeometryDrawing.Brush>
</GeometryDrawing>
<GeometryDrawing>
<GeometryDrawing.Geometry>
<RectangleGeometry Rect="100,0,100,100"/>
</GeometryDrawing.Geometry>
<GeometryDrawing.Brush>
<LinearGradientBrush StartPoint="1,0" EndPoint="0,1">
<GradientStop Offset="0.0" Color="Black" />
<GradientStop Offset="1.0" Color="White" />
</LinearGradientBrush>
</GeometryDrawing.Brush>
</GeometryDrawing>
<GeometryDrawing>
<GeometryDrawing.Geometry>
<RectangleGeometry Rect="0,100,100,100"/>
</GeometryDrawing.Geometry>
<GeometryDrawing.Brush>
<LinearGradientBrush StartPoint="0,1" EndPoint="1,0">
<GradientStop Offset="0.0" Color="Black" />
<GradientStop Offset="1.0" Color="White" />
</LinearGradientBrush>
</GeometryDrawing.Brush>
</GeometryDrawing>
<GeometryDrawing>
<GeometryDrawing.Geometry>
<RectangleGeometry Rect="100,100,100,100"/>
</GeometryDrawing.Geometry>
<GeometryDrawing.Brush>
<LinearGradientBrush StartPoint="1,1" EndPoint="0,0">
<GradientStop Offset="0.0" Color="Black" />
<GradientStop Offset="1.0" Color="White" />
</LinearGradientBrush>
</GeometryDrawing.Brush>
</GeometryDrawing>
</DrawingGroup.Children>
</DrawingGroup>
</DrawingBrush.Drawing>
</DrawingBrush>
</Window.Background>
ASCII-графика недостаточно выразительна, но, возможно, именно это вы имели в виду <<< >>>: