Restrict SelectionMode as Single

To restrict the selection mode to single, set the ListBoxSelectionMode options of GanttGrid’s Model in the GanttControl loaded event.

The following codes illustrate this

<gantt:GanttControl x:Name="Gantt"
                    ItemsSource="{Binding TaskDetails}"
                    VisualStyle="Metro"
                    Loaded="Gantt_Loaded">
</gantt:GanttControl>

private void Gantt_Loaded(object sender, RoutedEventArgs e)
{
    this.Gantt.GanttGrid.Model.Options.ListBoxSelectionMode = Syncfusion.Windows.Controls.Grid.GridSelectionMode.One;
}