It is open source, but doesn't have a developer ecosystem outside of google employees (in fact they outright state that no one else can be a committer). That in itself wouldn't be the end of the world, but unfortunately google has a) reduced resources devoted to GWT dramatically in the last year or so (many team members were reassigned to the dart project), and b) has a history rapidly changing 'paradigms' and semi-abandoning the code and documentation for the old way of doing things.