Try adding this. It is by no means a solution to the problem but if it works it may help us work out what is wrong with the original setup (add all of this at the end just before the smptClient.send command:
smtpClient.Port = 587
smtpClient.UseDefaultCredentials = FalsesmtpClient.Credentials = new System.Net.NetworkCredential ("YourUserName@gmail.com","YourGmailPassword")
smtpClient.EnableSsl = true