Ronald’s answer applies for Gmail accounts with two factor authentication in place. For a Gmail accounts without that, you need to use TLS and port 587. See screenshot below:
Note that when you initially set this up, your Mendix app will be denied SMTP access and you’ll get an email like this in your Gmail inbox:
After you click the button and confirm that it was you, you will also need to enable less secure app access in your Gmail account, which you can do by navigating to this URL: https://myaccount.google.com/lesssecureapps
Hope that helps.
You need app passwords for your own google account. Take a look at the google documentation here: https://support.google.com/accounts/answer/185833?hl=en
Regards,
Ronald