<sg:GcSpreadGrid Name="gcSpreadGrid1" ItemsSource="{StaticResource mySource}" 
           SelectionPolicy="Range" SelectionUnit="Cell" SelectionBackground="Red" 
           SelectionForeground="Blue" SelectionBackgroundUnfocused="Brown" SelectionForegroundUnfocused="Gray"
           HeaderSelectionOptions="ColumnHeader|RowHeader">
    <sg:GcSpreadGrid.SelectionBorder>
        <sg:BorderLine Color="Black" Style="MediumDashDot"/>
    </sg:GcSpreadGrid.SelectionBorder>
</sg:GcSpreadGrid>
<ComboBox ItemsSource="{Binding SelectedItems, ElementName=gcSpreadGrid1}" 
          SelectedItem="{Binding SelectedItem, ElementName=gcSpreadGrid1}"
          DisplayMemberPath="Name"/>