Scattered thoughts:

Nice idea. I stuck with Phonegap in the past, because I cared more about portability (including desktop/open web HTML) than features or profits for my little Set playalike game 3qual.

Which has a larger audience for games that fit the limitations (low power 2-D?): Java plus HTML5 plus Flash plus Android, or iOS? Or Windows/Metro?

If Google is serious about growing the N in playN, integrate with phonegap or equivalent to get onto iPhone and Windows Mobile too.

Does the GWT/Java replace HTML/JS/CSS (which would be a step backwards IMO) or host HTML/etc for UI? Or developer's choice?