I used the word "maintain" too loosely in the context - a better choice would have been "make conveniently available".
The library page is at http://code.google.com/apis/ajaxlibs
GWT development started several years before App Engine, but I think that its use with App Engine is about a lot more than branding.
For example, the new Google's new "Wave" product was developed using GWT and App Engine. These are complementary technologies (client side - server side) that are part of their overall software strategy.
App Engine has direct API's for the following Google services:
-- URL fetch
"The Google Data APIs Python module includes support for the following Google services:"
IMHO, they are positioning App Engine as the gateway for developers to integrate as many Google services as possible into applications.