<Page.Resources>
    <local:MyGroupStyleSelector x:Key="grpStyle"/>
</Page.Resources>
<GridView GroupStyleSelector="{StaticResource grpStyle}">
</GridView>

public class MyGroupStyleSelector : GroupStyleSelector
{
    protected override GroupStyle SelectGroupStyleCore(object item, uint level)
    {
        if( item as ClassType)
            return (GroupStyle)App.Current.Resources["grpStyle"];
            // use condition to chk the type of Group style u want to select from the one defined in xaml
        }
    }
}