Sorry but all the Bitnami images launched through our Launchpads include all the components the application needs to run.
That means that for every deployment, you will have the application and all its requirements (database, web server, ...) installed and configured in the same server. This configuration doesn't allow to use a load balancing or adding more applications to the instance group as the different deployments will use different databases.
If you want to use that architecture, you will need to separate the database from the applications (or use the database of one deployment) and configure the rest of servers to use that database instead of using the local one. Then, you can follow the Google documentation to configure the instance group with those instances.
You can also launch a Multi-Tier solution in Google using Bitnami. Google Multi-Tier Solutions powered by Bitnami allow you to deploy server cluster configurations and horizontal scalable applications. Bitnami provides ready-to-use Multi-Tier Solutions for Google Cloud Platform servers, so that you can get productive with your new application immediately. You can access and launch these Multi-Tier Solutions using the Google Cloud Launcher.
I hope this information helps. Regards,