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 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 too. Leave it blank to use the default storage class.
The configured bucket must exist.
This backend is very similar to the S3 backend, and it has the same limitations. As with S3 chtime will fail with the default configuration, you can install the metadata plugin to make it work and thus be able to preserve/change file modification times.