According to https://cloud.google.com/storage/docs/deleting-objects#delete-objects-in-bulk one should use "lifecycle configuration rules":

To bulk delete objects in your bucket using this feature, set a lifecycle configuration rule on your bucket where the condition has Age set to 0 days and the action is set to delete . Once you set the rule, Cloud Storage performs the bulk delete asynchronously.

To just delete a directory, "prefix" can be used.