Setting up a G Suite (formally Google Apps) is easy in Ghost. Simply configure G Suite to allow the IP address of the server in the SMTP relay service settings. Remember to require TLS encrpytion too.

To finish simply set the following mail settings in your instances config.js

mail: {
          transport: 'SMTP',
          options: {
            host: 'smtp-relay.gmail.com',
            port: 587,
          }

    },