Amazon DynamoDB and Azure Table Storage are already supported, now GCP is missing for support of the "big three" cloud providers.
Note: Cloud Firestore (not Firebase) has a "Datastore mode" which makes it compatible to the Datastore API. Firestore might superseed Datastore in the future. But it's currently marked as beta:
Amazon DynamoDB and Azure Table Storage are already supported, now GCP is missing for support of the "big three" cloud providers.