You shouldn't have to put it in ViewState. Revert back to the following, but add the first line that I have below and let me know what the output it produces is:

protected void gridView_Sorting(object sender, GridViewSortEventArgs e)
{
   Response.Write(gridView.DataSource.GetType()); //Add this line

   DataTable m_DataTable = gridView.DataSource as DataTable;

   if (m_DataTable != null)
   {
      DataView m_DataView = new DataView(m_DataTable);
      m_DataView.Sort = e.SortExpression + " " + ConvertSortDirectionToSql(e.SortDirection);

      gridView.DataSource = m_DataView;
      gridView.DataBind();
   }
}