Private Function GetData() As DataTable
    Dim constr As String
    Dim connection As SqlConnection
    constr = ConfigurationManager.ConnectionStrings("Connectionstring").ToString
    connection = New SqlConnection(constr)
    Using con As SqlConnection = New SqlConnection(constr)
        Using cmd As SqlCommand = New SqlCommand("SELECT * FROM customers")
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using dt As DataTable = New DataTable()
                    sda.Fill(dt)
                    Return dt
                End Using
            End Using
        End Using
    End Using
End Function

Protected Sub exportexcel() '

    Dim dt As DataTable = GetData()
    dt.TableName = "GridView_Data"

    Using wb As XLWorkbook = New XLWorkbook()
        wb.Worksheets.Add(dt)
        Using memoryStream As MemoryStream = New MemoryStream()
            wb.SaveAs(memoryStream)
            Dim bytes As Byte() = memoryStream.ToArray()
            memoryStream.Close()
            Using mm As MailMessage = New MailMessage("sender@gmail.com", "receiver@gmail.com")
                mm.Subject = "GridView Exported Excel"
                mm.Body = "GridView Exported Excel Attachment"
                mm.Attachments.Add(New Attachment(New MemoryStream(bytes), "GridView.xlsx"))
                mm.IsBodyHtml = True
                Dim smtp As SmtpClient = New SmtpClient()
                smtp.Host = "smtp.gmail.com"
                smtp.EnableSsl = True
                Dim credentials As New NetworkCredential("xxxxxx@xxxxx.com", "xxxxxxxx")

                smtp.UseDefaultCredentials = True
                smtp.Credentials = credentials
                smtp.Port = 587
                smtp.Send(mm)
            End Using
        End Using
    End Using
End Sub

Protected Sub Button2_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button2.Click
    'ExportExcel()
    If Not Me.IsPostBack Then
        GridView1.DataSource = Me.GetData()
        GridView1.DataBind()
        exportexcel()
    End If
End Sub