I use dapfor and this code underneath work perfectly for the question you asked.
XAML
<!--Header group panel brushes-->
<SolidColorBrush x:Key="GroupPanelForegroundBrush" Color="#ffdae0e7"/>
<LinearGradientBrush x:Key="GroupPanelBackgroundBrush" StartPoint="0,0" EndPoint="1,1">
<GradientStop Color="#00ffffff" Offset="0" />
<GradientStop Color="#20ffffff" Offset="0.2" />
<GradientStop Color="#00ffffff" Offset="0.3" />
<GradientStop Color="#40ffffff" Offset="0.4" />
<GradientStop Color="#00ffffff" Offset="0.5" />
<GradientStop Color="#20ffffff" Offset="0.6" />
<GradientStop Color="#40ffffff" Offset="0.7" />
<GradientStop Color="#00ffffff" Offset="0.8" />
<GradientStop Color="#20ffffff" Offset="0.9" />
<GradientStop Color="#00ffffff" Offset="1" />
</LinearGradientBrush>
<LinearGradientBrush x:Key="GroupPanelBorderBrush" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#40ffffff" Offset="0" />
<GradientStop Color="#35000000" Offset="1" />
</LinearGradientBrush>

<!--GridControl-->
<Style TargetType="{x:Type controls:GridControl}">
<Setter Property="AppearanceHeaderGroupPanelBackground" Value="{StaticResource GroupPanelBackgroundBrush}"/>
<Setter Property="AppearanceHeaderGroupPanelForeground" Value="{StaticResource GroupPanelForegroundBrush}"/>
<Setter Property="AppearanceHeaderGroupPanelBorder" Value="{StaticResource GroupPanelBorderBrush}"/>
<Setter Property="AppearanceHeaderGroupPanelBorderThicknes s" Value="1"/>
</Style>