As stated, you're limited by the language. GWT client side code only currently supports jdk 1.4 and even that is very limited in the JRE emulation. Check out this link that shows you the javadocs for the classes that are supported (http://code.google.com/webtoolkit/documentation/jre.html). Very limited indeed.
But with that said, I haven't had any problems writing a fairly decent size GWT application for our project. Pretty much if I can't do anything on the client side because of the limited emulation, I wrap the call in an RPC invocation and do it on the server side, which is full jdk 1.5 (or even 1.6) supported.