http://code.google.com/web/ajaxcrawling/docs/html-snapshot.html
Does HtmlUnit work on AppEngine? If not, are there any other ways to make my GWT app crawlable by search engines?
A patch for HtmlUnit to work on GAE is in progress. HtmlUnit's bug tracker issue 2962074 discusses making HtmlUnit work on GAE, and provides a preliminary patch for accomplishing this.