According to Google, the settings for sending mail through Gmail are:

This is the way we recommend as it uses current Internet standards.

1. Go to Mail Connectors → Mail Senders and edit your mail sender.
2. In the Mail Send Method box, make sure you have chosen Send via an SMTP Relay (Smarthost).
3. Go to the Settings tab
4. In the SMTP Relay Servers box, type smtp.gmail.com:587
5. Tick the This server requires SMTP authentication box.
6. In the SMTP Username/Password boxes type your Gmail username & password.
7. In the Session Encryption box, choose either TLS if available or TLS
8. Press Submit

This is not the way we recommend, but it will work if you need it

1. Go to Mail Connectors → Mail Senders and edit your mail sender.
2. In the Mail Send Method box, make sure you have chosen Send via an SMTP Relay (Smarthost).
3. Go to the Settings tab
4. In the SMTP Relay Servers box, type smtp.gmail.com:465
5. Tick the This server requires SMTP authentication box.
6. In the SMTP Username/Password boxes type your Gmail username & password.
7. In the Session Encryption box, choose SSL
8. Press Submit