To connect SFTPGo to Google Cloud Storage you can use use the Application Default Credentials (ADC) strategy to try to find your application's credentials automatically or you can explicitly provide a JSON credentials file that you can obtain from the Google Cloud Console. Take a look [here](https://cloud.google.com/docs/authentication/production#providing_credentials_to_your_application) for details.
Specifying a different `key_prefix`, you can assign different "folders" of the same bucket to different users. This is similar to a chroot directory for local filesystem. Each SFTP/SCP user can only access the assigned folder and its contents. The folder identified by `key_prefix` does not need to be pre-created.
You can optionally specify a [storage class](https://cloud.google.com/storage/docs/storage-classes) too. Leave it blank to use the default storage class.