Like the OP I have tried every settings that I could think of and still wasn't able to get SuiteCRM on Google Cloud Platform (GCP) to send a test email.
Using your configuration, I received the following error:
Error:SMTP connect() failed.
Gmail account is only protected by a password with 2-Step Verification disabled. Clicking on send a test email button, I'd receive an email shortly after from Google with the message "Sign-in attempt prevented". This is happening even with minimal security.
Bitnami Google Apps - How To Configure Outbound Email Settings?
Google Cloud Platform doesn't allow SMTP traffic through default ports: 25, 465, 587. 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.
Google Cloud Platform - Sending Email from an Instance
If you don't have a Google Apps account or don't want to use Google Apps or a third-party mail provider, you can set up your own email server on an instance using a non-standard port. You can choose any ephemeral port that isn't blocked by Compute Engine.
Looks like my option now are to create VPN or mail server using non-standard port. This is time consuming trying to get this working.