sudo ./bnsupport-tool runSMTPTool 1 should produce extra files with SMTP info, but it is not present in the output you sent.
Anyway, I didn't share with you the right link. Since you are use GCE, it is:
As you can see there, Google Cloud Platform doesn’t allow SMTP traffic through default ports: 25. Check Google cloud documentation to learn how to use a VPN to bypass these restrictions or use a different port for sending emails from your application.
If you continue facing issues, please share your SMTP configuration with us? About your credentials, it will be enough with knowing your email domain.