public class GMail
{
    public void SendMailFromGoogle()
    {
        MailMessage mail = new MailMessage();
        mail.From = new MailAddress("sender@mail.com");
        mail.To.Add("receiver@mail.com");
        mail.Subject = "Email Subject";
        mail.Body = "Email Body";
        var attachment = new Attachment("C:\\path\\file.ext");
        mail.Attachments.Add(attachment);

        SmtpClient smtp = new SmtpClient("smtp.gmail.com");
        smtp.Port = 587;

        // Account Username: Usually your "from" email.
        // App Password: If your account has a 2 step verification, you must follow these instructions 
        //  to generate the password: https://support.google.com/accounts/answer/185833?hl=en.
        smtp.Credentials = new NetworkCredential("Account Username", "App Password") as ICredentialsByHost;
        smtp.EnableSsl = true;

        ServicePointManager.ServerCertificateValidationCallback =
                delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) {
                    return true;
                };

        smtp.Send(mail);
    }
}