smtp.Credentials = new System.Net.NetworkCredential("ggcobani@gmail.com", "password");
smtp.EnableSsl = true;
smtp.UseDefaultCredentials = false;
Setting UseDefaultCredentials to false clears any credentials you have previously set, so swap those statements around
smtp.UseDefaultCredentials = false;
smtp.Credentials = new System.Net.NetworkCredential("ggcobani@gmail.com", "password");
smtp.EnableSsl = true;
Sending email through gmail is probably one of the most frequently asked questions there is. There are countless code samples and troubleshooting guides out there, no matter what your problem you'll find a solution by googling it.