How to override GroupBarItemHeader Font in WPF GroupBar?
You can change the font style of the header text in the GroupBar control, by customizing the HeaderTemplate in the ItemContainerStyle property.
Refer the below code for your reference.
XAML
<syncfusion:GroupBar Height="130" MinWidth="150" MinHeight="200" VisualMode="StackMode" Width="200" >
<syncfusion:GroupBar.ItemContainerStyle>
<Style TargetType="syncfusion:GroupBarItem">
<Setter Property="HeaderTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock Text="{Binding Text}" FontSize="15" FontFamily="Algerian" />
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
</syncfusion:GroupBar.ItemContainerStyle>
<syncfusion:GroupBarItem ContentBreadth="0" ContentLength="0" HeaderText="Gmail" ShowInGroupBar="True">
<syncfusion:GroupView IsListViewMode="True">
<syncfusion:GroupViewItem Text="Inbox"/>
</syncfusion:GroupView>
</syncfusion:GroupBarItem>
<syncfusion:GroupBarItem ShowInGroupBar="True" HeaderText="Messages">
<syncfusion:GroupView IsListViewMode="True">
<syncfusion:GroupViewItem Text="Send / Receive" />
</syncfusion:GroupView>
</syncfusion:GroupBarItem>
</syncfusion:GroupBar>
