I've collected some info of Gather API calls the Android app could use to sync saved pages. This could be extended for general collections and watch lists later.
There are some features missing from the Gather API and deployments to other wikis besides en and he WP. See my comments on https://phabricator.wikimedia.org/T123377 for details. Add your own comments if you have ideas for improvement or can help fix any of the current limitations of the API.
Thanks, Bernd