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.
That's interesting. I've done my fair share of GWT development, and I didn't know about this, so I tried to find more info.
I've searched through the GWT source code, and I found no mention of the closure library (closure tools / closure compiler). There are obviously some mentions of javascript "closures", but that's all I found.
Recently, I have been using either GWT or SmartGWT much more often than manually writing Javascript. I was curious if Google was consuming their own dog food. For casual UI developers like me (I specialize in other things) GWT really makes it easier to get stuff done.
I would be curious about how often GWT is used in small internal Google projects. From reading Mark Chu-Carroll's (very useful) book "Code in the Cloud" I got the impression that he liked to use GWT when he has to do any UIs. He works at Google.
They do eat their own dog food. They say they use it in AdWords and Orkut. But not here it doesn't appear.
I use GWT every time I write JS if I can help it because the static typing and large project management features of Java are nice (hell, I even wrote a wrapper for node.js in GWT)
