Got a invite mail yesterday night from the GME team. Was waiting for it all this while. Started tinkering with it. The mashup editor provides a lil web-based IDE to create, test and publish our mashups.
GME provides tags [both UI related and data related] like gm:page, gm:tabs, gm:list & of course HTML, CSS and JavaScript.
GME also provides a Javascript API via which we get direct access to objects in the application and also manipulate [Create, Read, Update & Delete ] data feed entries. It is possible to perform customized event handling for all application components.
You can find recent entry titles of my two blogs in this lil mashup.