What is GWT?
GWT apps are developed in Java. It definitely does not require many years of experience as a Java developer to start with GWT. But you should have a basic understanding of the Java-language and know how anonymous inner classes work since all the event handling uses them heavily. Check out this tutorial for more information. If you have some experience developing Swing-applications: perfect. You'll recognize many of these concepts are used in GWT.
Using Eclipse is the most convenient way of developing GWT apps in Windows, OSX or Linux.
With a lot of time you will probably be able to get it running in any other IDE like Netbeans, BlueJ or IntelliJ. And of course you can also use the command-line version with a text editor.
Since most folks at Google seem use Eclipse or BBEdit I think that you are best of by just doing the same in order to run into the least problems.
HTML and CSS
GWT apps are layouted using CSS and ambedded into HTML pages. Therefore you need some basic experience with CSS and HTML to get it running. The standard layout looks like - well - "basic". If you want something more advanced you have to style it yourself with CSS. Unfortunately I have not seen a comprehensive collection of good CSS examples for GWT yet. Drop me a note if you find something.
Firebug can make your life a lot easier finding the problems with CSS.