Have no first-hand experience, but someone asked this question of Chris Sacca (Google) at the first startup school, and he said:
"Don't worry about it. Use whatever you're most comfortable developing in. If we buy you, we'll likely have to rewrite all your code anyway to use Google's infrastructure and scale to millions of users."
They'll still have to rewrite. The "infrastructure" he was referring to was GoogleFS/BigTable/MapReduce and their other massively-distributed backends. You don't have access to them if you aren't Google.
I've heard that GWT isn't actually used by any notable (= public has heard of it) Google project. Could be wrong though; no inside information.
"Don't worry about it. Use whatever you're most comfortable developing in. If we buy you, we'll likely have to rewrite all your code anyway to use Google's infrastructure and scale to millions of users."