Hi Benny,
In my opinion, it is essential to create custom authentication instead of using Google Account API to create any viable application run on GAE.
So, I created my own solution to tackle this problem which you may interested to have a look at URL:
http://grailsfuse.vobject.com/
(You will hit http 500 error for first request, please wait for 30 seconds and refresh)
The missing part is the self-registration page and remember me feature. Please contact us (hyperlink located below the page of URL above) if you are still interested.
Wish to heard from you soon!