Hello Olivier,
The "normal" way web sites work is a two step procedure:
1. The browser sends a request
2. The server sends a response which replaces the HTML page the
browser currently displays
So, after 2 has finished, nothing is left of the old HTML page. It is
erased and a new content is displayed to the user.
With RPC the procedure changes:
1. The browser sends a request
2. The server sends a response which the javascript in the browser
receives, parses and reacts to it.
So, all you can ever get from the server as a response is a java
object which your program must receive and react to.
Now, if you want to display a JSP page after an RPC call, you still
can change the window location (see the javadocs for GWT's Window
class) to direct you to that JSP.