Below class will help us in exporting the gridView data to the excel file.

  public class clsExportToExcel
    {

        public static void ExportDataSetToExcel(DataTable dTable, string filename)
        {
            HttpResponse response = HttpContext.Current.Response;
            response.Clear();
            response.Charset = "";
            response.ContentType = "application/vnd.ms-excel";
            response.AddHeader("Content-Disposition", "attachment;filename=\"" + filename + "\"");
            response.Cache.SetCacheability(HttpCacheability.NoCache);
            using (StringWriter sw = new StringWriter())
            {
                using (HtmlTextWriter htw = new HtmlTextWriter(sw))
                {
                    DataGrid dg = new DataGrid();
                    dg.DataSource = dTable;
                    dg.DataBind();
                    dg.RenderControl(htw);
                    response.Write(sw.ToString());
                    response.End();
                }
            }
        }
    }