Whenever a new Google site comes out, everyone always asks this question. I can only assume it is asked by people who only know Java and are terrified of JS (which seems to be GWT's primary audience, despite the framework's other advantages).

If you want to know, just pop open Firebug or Chrome's Inspector and search for the string "__gwt". I don't see it in Google+.

To my knowledge GWT is not actually used that much within Google. Most of the things it solves for developers are solved in other ways within Google anyway, and if you want really low-level control GWT is not the best option.