Could you please let us know if you changed the permissions of the home folder of the bitnami user?
As you can see in this guide, you can add a new SSH key to the bitnami user and try to access the server. If that doesn't work, you probably modified the .ssh folder permissions and that can break the connection. If that's the case, you will need to set a startup-script in Google to recover the permissions of that folder.
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