Following up to this thread since I can't comment.
I've successfully created object change notifications as instructed by @Brandon Yarbrough (thanks!).
But I would like to narrow down the monitoring from the entire bucket to a sub-folder. For example, from 'gs://mainbucket/' to 'gs://mainbucket/subfolder1/'. However, attempting this in the same syntax returns this error.
CommandException: notification create requires a GCS bucket name, but got 'gs://mainbucket/subfolder1/'
How should this function be implemented instead?