1. what is the advantage of using GWT when compared to ordinary Ajax implementation? (or) why GWT?
2.I felt there are some difference in implementing Ajax in PHP and JSP, Is both languages GWT implementations are covered in this book?
You are talking about the server-side here. GWT has its own proprietary RPC mechanism (GWT-RPC), where you would use servlets on the server.
Besides this you can use alternative methods, like REST, XML messages, and JSON. You can use whatever you want on the server. Some people use Struts, some Spring, some PHP, Perl, Ruby, etc, etc.
GWT is server agnostic.
3.How is AJAX JSP Tag Library, Is GWT similar to that?
No tags on the server. GWT is all about the code on the client-side. The only actual code it provides for the server is the GWT-RPC tool. There are a bunch of third-party projects out there though, like one for JSF integration, Spring integration, etc.