You can try the following code block instead of your code.
DataSet ds = new DataSet();
ds=obj_bal.BAL_ShowFrom_Reply();
GridView1.DataSource = ds;
GridView1.DataBind();
Response.Clear();
Response.ClearHeaders();
Response.AddHeader("content-disposition", "attachment;filename=test.docx");
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.ContentType = "application/vnd.word";
System.IO.StringWriter stringWrite = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
HtmlForm frm = new HtmlForm();
GridView1.Parent.Controls.Add(frm);
frm.Attributes["runat"] = "server";
frm.Controls.Add(GridView1);
frm.RenderControl(htmlWrite);
Response.Write(stringWrite.ToString());
Response.End();