I'm a newbie to GWT. I mean just started learning it. So I want to ask that for a large scale project (web app ike Facebook) is GWT preferable? Why? We are confused whether to go for pure JSP/servlets or GWT like framework? Thanking in advance....
GWT is the Java wrapper for Ajax. If your requirement dictates you refresh part(s) of your page content periodically, go for GWT, because thats where Ajax really helps you.
Gmail does not use GWT. A list (dated though) of applications built with GWT can be found here http://code.google.com/webtoolkit/doc/1.6/whos_using_gwt.html. Googling will give you more.
GMail uses Google Closures, Google Waves was written in GWT.
GWT basically is about coding your client-side logic in Java programming language, you are free to use any language/technology on the server side.
Your scalability requirements depends on you back end, not on GWT.