O101 giving internet access initially to download the models
OS will not be able to control what the app uses internet for - be it download or seeding data about your system and environment to Google so they can "provide seamless and better user experience".
You can try to use rethinkdns to whitelist particular domains, assuming the app uses them only for download of offline datasets, but you will need to figure them out yourself.
An example may be Google translate. It downloads offline dictionaries from dl.google.com but contacts a lot of other domains for other activities.
If you whitelist only dl.google.com, GT app will work (not the best experience though) in offline mode. However there is no guarantee that it doesn't share extra info via dl.google.com
You can play with rethinkdns on your non-gos phone if you have one.