If the object you want to put this background on is a border or a grid, you can just add a Path
object behind the rest of your content in that control:
<Border Width="100" Height="50">
<Path Data="m0,0 L 0 50 L 100 50 L 100 30 C 70 0 30 25 0 0">
<Path.Fill>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0.3" Color="Green"/>
<GradientStop Offset="1" Color="#FF99bb99"/>
</LinearGradientBrush>
</Path.Fill>
</Path>
</Border>