maintaining leading zeros when exporting to excel

How do I do that with the code you helped me out with Mudassar?

protected void btnExportExcel_Click(object sender, EventArgs e)
{
    Response.Clear();
    Response.Buffer = true;
    
    Response.AddHeader("content-disposition", 
     "attachment;filename=GridViewExport.xls");
    Response.Charset = "";
    Response.ContentType = "application/vnd.ms-excel";
    StringWriter sw = new StringWriter();
    HtmlTextWriter hw = new HtmlTextWriter(sw);
 
    GridView1.AllowPaging = false;
    GridView1.DataBind(); 
 
    //Change the Header Row back to white color
    GridView1.HeaderRow.Style.Add("background-color", "#FFFFFF");
 
    //Apply style to Individual Cells
    GridView1.HeaderRow.Cells[0].Style.Add("background-color", "green");
    GridView1.HeaderRow.Cells[1].Style.Add("background-color", "green");
    GridView1.HeaderRow.Cells[2].Style.Add("background-color", "green");
    GridView1.HeaderRow.Cells[3].Style.Add("background-color", "green");   
 
    for (int i = 0; i < GridView1.Rows.Count;i++ )
    {
        GridViewRow row = GridView1.Rows[i];
 
        //Change Color back to white
        row.BackColor = System.Drawing.Color.White;
 
        //Apply text style to each Row
        row.Attributes.Add("class", "textmode");
 
        //Apply style to Individual Cells of Alternating Row
        if (i % 2 != 0)
        {
            row.Cells[0].Style.Add("background-color", "#C2D69B");
            row.Cells[1].Style.Add("background-color", "#C2D69B");
            row.Cells[2].Style.Add("background-color", "#C2D69B");
            row.Cells[3].Style.Add("background-color", "#C2D69B");   
        }
    }
    //Remove Controls
    this.RemoveControls(GridView1);
    GridView1.RenderControl(hw);
 
    //style to format numbers to string
    string style = @"<style> .textmode { mso-number-format:\@; } </style>"; 
    Response.Write(style); 
    Response.Output.Write(sw.ToString());
    Response.Flush();
    Response.End();
}
 
private void RemoveControls(Control grid)
{
    Literal literal = new Literal();
    for (int i = 0; i < grid.Controls.Count; i++)
    {
        if (grid.Controls[i] is LinkButton)
        {
            literal.Text = (grid.Controls[i] as LinkButton).Text;
            grid.Controls.Remove(grid.Controls[i]);
            grid.Controls.AddAt(i, literal);
        }
        else if (grid.Controls[i] is DropDownList)
        {
            literal.Text = (grid.Controls[i] as DropDownList).SelectedItem.Text;
            grid.Controls.Remove(grid.Controls[i]);
            grid.Controls.AddAt(i, literal);
        }
        else if (grid.Controls[i] is CheckBox)
        {
            literal.Text = (grid.Controls[i] as CheckBox).Checked ? "True" : "False";
            grid.Controls.Remove(grid.Controls[i]);
            grid.Controls.AddAt(i, literal);
        }
        else if (grid.Controls[i] is HyperLink)
        {
            literal.Text = (grid.Controls[i] as HyperLink).Text;
            grid.Controls.Remove(grid.Controls[i]);
            grid.Controls.AddAt(i, literal);
        }
        else if (grid.Controls[i] is Button)
        {
            literal.Text = (grid.Controls[i] as Button).Text;
            grid.Controls.Remove(grid.Controls[i]);
            grid.Controls.AddAt(i, literal);
        }
        if (grid.Controls[i].HasControls())
        {
            RemoveControls(grid.Controls[i]);
        }
    }
}