I guess this is the bitnami image, so not supported here.
I’m running a discourse instance on google cloud, just follow the official guide and everything works fine.
If you launched it from Google Cloud Launcher, then it is a Bitnami image.
https://console.cloud.google.com/launcher/details/bitnami-launchpad/discourse?q=discourse
Quoting StackOverflow answer:
Google Cloud Platform doesn’t allow SMTP traffic through default ports: 25, 465, 587. You can check Google Cloud Documentation to configure a third-party mail provider like Sendgrid, Mailgun or Mailjet. If you use Google Apps you can also use it to send email through 465 or 587 ports.
Another alternative could be that you use your own email server on a custom port or using a VPN to bypass that port restriction.
You can find more info on Bitnami Wiki page for Google Cloud Platform
If you find more issues with the Discourse Bitnami image or installer, you can post them in our “Discourse powered” community forum at https://community.bitnami.com 
Regards,
Jorge
Disclaimer: I work at Bitnami
Send Grid would be great, I have an account there. How do I set it up on my Discourse install?
Check this out and try to fill all the fields with your Sendgrid parameters:
https://wiki.bitnami.com/Applications/BitNami_Discourse#How_to_configure_SMTP_settings_to_send_emails_from_Discourse_application.3f