To send mail to Gmail using System::Net::Mail::SmtpClient and SSL, you must use port 587, as documented in the GMail API.
Both ports 25 and 587 are available on GMail, but Port 587 will require a SMTP authentication before sending a mail, while port 25 won't. Given that port 25 don't require sender authentication, GMail will more agressively filter connections from ip adresses which are not proper mail servers in order to reduce spams.
The GMail API also allows use of port 465, but you can't use it with System::Net::Mail::SmtpClient, as documented by Microsoft. Port 465 is for SMTP over SSL : first establish a SSL connection, then execute the SMTP transaction. With ports 25 and 587, an unencrypted SMTP session is opened first, before switching to SSL using STARTTLS and completing the SMTP transaction. The latter is the method implemented by the .Net SMTP client.