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.
has a list - just eyeballing it, it seems like about half of the major javascript apps from google are GWT. (the other half would likely be closure)