Overriding data grid columns in Gantt Chart Light Library

How can I define an overriding set of columns within a DataGrid-based control of Gantt Chart Light Library?

You need to define a full column collection as the Columns property value:

<pdgcc:GanttChartDataGrid.Columns>
    <pdgcc:DataGridColumnCollection>
        <hd:DataTreeGridColumn Header="Task"/>
        <dataControls:DataGridTextColumn Header="Start" Binding="{Binding Start, Mode=TwoWay, Converter={StaticResource DateTimeStringConverter}}"/>
        <dataControls:DataGridTextColumn Header="Finish" Binding="{Binding Finish, Mode=TwoWay, Converter={StaticResource DateTimeStringConverter}}"/>
    </pdgcc:DataGridColumnCollection>
</pdgcc:GanttChartDataGrid.Columns>

Note that without adding the collection container tag, the specified columns would be just added to the existing collection (i.e. after the default, previously existing, columns).