public class MyGridViewTextBoxColumn : GridViewTextBoxColumn
{
    protected override GridViewColumnValuesCollection GetDistinctValuesWithFilter()
    {
        int index = this.Index;
        bool containsBlanks = false;
        GridViewColumnValuesCollection distinctValues = new GridViewColumnValuesCollection();

        IEnumerable<GridViewRowInfo> rows = this.OwnerTemplate.Rows;

        foreach (GridViewRowInfo row in rows)
        {
            object cellValue = row.Cells[index].Value;
            if (!containsBlanks)
            {
                containsBlanks = cellValue == null || cellValue == DBNull.Value;
            }
            if (cellValue == "")
            {
                cellValue = "(Blanks)";
            }
            if (!distinctValues.Contains(cellValue))
            {
                distinctValues.Add(cellValue);
            }
        }
        return distinctValues;
    }
}