Can anyone point me to a useful guide for UI designers working in google web toolkit?
Well if your looking on how to use widgets and panels in gwt then i would recommend Roughian Examples This website provides us the basic usage of the GWT widgets and panels and provides us with enough information just to get things started.
As per my comment to bhargava's answer, your designers should be learning UiBinder. The whole chapter about building user interfaces seems appropriate too (to get a better perspective), but UiBinder is what they will be mostly dealing with. Without using UiBinder in your project, you are stuck with Java and that's not something your designers are likely to know (and are probably not keen to :)).
I'd recommend building a simple example (but not too simple - maybe you should "strip" the official mail example) that uses UiBinder and show the designers exactly what you expect them to provide and what should be left as stubs. It all depends on the designers in question - whether all they know is HTML and CSS or maybe they have experience with Java, etc. Tailor the example to your needs - you probably won't "get it right" the first time, but with feedback from the designers (what's hard to understand, what they think should be the responsibility of the programmer, etc.), you should arrive at a good learning tool for future employees and a reference for current ones :)