Hi gibson,

I have created a sample which full fill your requirement you need to the code according to your requirement.

HTML

<div>
    <asp:GridView ID="gvCustomers" runat="server" />
    <br />
    <asp:Button ID="btnExport" Text="ExportToExcel" runat="server" OnClick="btnExport_Click" />
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        BindGrid();
    }
}

protected void btnExport_Click(object sender, EventArgs e)
{
    DataTable dt = new DataTable();

    for (int i = 0; i < gvCustomers.HeaderRow.Cells.Count - 1; i++)
    {
        dt.Columns.Add(gvCustomers.HeaderRow.Cells[i].Text.ToString());
    }

    foreach (GridViewRow row in gvCustomers.Rows)
    {
        DataRow dr = dt.NewRow();
        dr["CustomerID"] = row.Cells[0].Text;
        dr["CompanyName"] = row.Cells[1].Text;
        dr["ContactName"] = row.Cells[2].Text;
        dr["City"] = row.Cells[3].Text;
        dr["Country"] = row.Cells[4].Text;
        dt.Rows.Add(dr);
    }

    //Create a dummy GridView
    GridView GridView1 = new GridView();
    GridView1.AllowPaging = false;
    GridView1.DataSource = dt;
    GridView1.DataBind();
    Response.Clear();
    Response.Buffer = true;
    Response.AddHeader("content-disposition", "attachment;filename=Customers.xls");
    Response.Charset = "";
    Response.ContentType = "application/vnd.ms-excel";

    StringWriter sw = new StringWriter();
    HtmlTextWriter hw = new HtmlTextWriter(sw);

    for (int i = 0; i < GridView1.Rows.Count; i++)
    {
        //Apply text style to each Row
        GridView1.Rows[i].Attributes.Add("class", "textmode");
    }

    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 BindGrid()
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "SELECT TOP 10 CustomerID,CompanyName,ContactName,City,Country,PostalCode FROM Customers";
            cmd.Connection = con;
            con.Open();
            gvCustomers.DataSource = cmd.ExecuteReader();
            gvCustomers.DataBind();
            con.Close();
        }
    }
}

public override void VerifyRenderingInServerForm(Control control)
{
    /* Verifies that the control is rendered */
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
	If Not IsPostBack Then
		BindGrid()
	End If
End Sub

Protected Sub btnExport_Click(sender As Object, e As EventArgs)
	Dim dt As New DataTable()

	For i As Integer = 0 To gvCustomers.HeaderRow.Cells.Count - 2
		dt.Columns.Add(gvCustomers.HeaderRow.Cells(i).Text.ToString())
	Next

	For Each row As GridViewRow In gvCustomers.Rows
		Dim dr As DataRow = dt.NewRow()
		dr("CustomerID") = row.Cells(0).Text
		dr("CompanyName") = row.Cells(1).Text
		dr("ContactName") = row.Cells(2).Text
		dr("City") = row.Cells(3).Text
		dr("Country") = row.Cells(4).Text
		dt.Rows.Add(dr)
	Next

	'Create a dummy GridView
	Dim GridView1 As New GridView()
	GridView1.AllowPaging = False
	GridView1.DataSource = dt
	GridView1.DataBind()
	Response.Clear()
	Response.Buffer = True
	Response.AddHeader("content-disposition", "attachment;filename=Customers.xls")
	Response.Charset = ""
	Response.ContentType = "application/vnd.ms-excel"

	Dim sw As New StringWriter()
	Dim hw As New HtmlTextWriter(sw)

	For i As Integer = 0 To GridView1.Rows.Count - 1
		'Apply text style to each Row
		GridView1.Rows(i).Attributes.Add("class", "textmode")
	Next

	GridView1.RenderControl(hw)

	'style to format numbers to string
	Dim style As String = "<style> .textmode { mso-number-format:\@; } </style>"
	Response.Write(style)
	Response.Output.Write(sw.ToString())
	Response.Flush()
	Response.[End]()
End Sub

Private Sub BindGrid()
	Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
	Using con As New SqlConnection(constr)
		Using cmd As New SqlCommand()
			cmd.CommandText = "SELECT TOP 10 CustomerID,CompanyName,ContactName,City,Country,PostalCode FROM Customers"
			cmd.Connection = con
			con.Open()
			gvCustomers.DataSource = cmd.ExecuteReader()
			gvCustomers.DataBind()
			con.Close()
		End Using
	End Using
End Sub

Public Overrides Sub VerifyRenderingInServerForm(control As Control)
	' Verifies that the control is rendered 
End Sub

ScreenShot