Hi Jocelyn,
DAGs in Cloud Composer can read/write data to any GCS bucket as long as:
- if you don't specify a connection id, please make sure the service account of your Composer environment is configured with read/write access to the desired bucket.
- if you do specify a GCP connection, similarly you'll need to ensure the service account in the GCP connection has access to the bucket.
Looking at the error message you shared, it appears that your DAG is broken (i.e., python code problem) and ran into undefined variable 'gcs'.
Feng