I would like to develop a web-app requiring data persistence using GWT and GAE. As I understand it, my only (or at least by far the most convenient) option for data persiste
You can consider using JSON. GWT has necessary API to parse & generate JSON string in the client side. You get a lot of JSON API for server side. I tried with google-gson, which is fine. It converts your JSON string to POJO model and viceversa. Hope this helps you providing a decent solution for your requirement