GWT does not require an advance level of java knowledge, but you do need to have some knowledge of java GUI development (unless you use the ui xml pattern which requires some knowledge of html). You should also have some idea of how web technologies and ajax works.
Hi! When we hire people for GWT programming, if they do not already know it, we ask for Java+Swing knowledge. It's quite simple going from there to GWT development. Also, it helps if the programmer knows Ajax, because then he is already aware of the client/server communications details.
Hope this helps!
Great fan of Open Source, Linux, and web development with GWT; all of these come together for my ESSENTIAL GWT book!
As Federico said, there are a lot of similiarities to Swing (or other desktop component frameworks). I think the result from the architecture is that a GWT programmer needs a deeper understanding for what he does.
When writing a (simple) page based web application (PHP, JSP only...) there are not that much places where a programmer can do serious mistakes. After the page is rendered, no fragments should remain, the request is closed, the client received some HTML, ideally no objects left.
When there is server-side state involved (e.g. Wicket) then accidentily serialising large objects (like data read from a DB... or the whole Spring context) can kill the server.
When using a technology that allows you to send Java objects to the client... unexperienced programmers may do this as it is easy. Just think of sending the User object directly from the database to the client... so a malicious client could read all the serialised fields - password hash, private states, internal ids... and it may even look clever at a first glance because you don't have to create extra DTOs. Plus you still have a server side to code, with an extra level of security risk.
So I think the more "state" there is the more insight a programmer needs. Client side state, whether fat client or something like GWT, adds an additional layer of risk and potential trouble - and in consequence requires more experience. Page based just is easier.