<p>I m just starting using gwt and so far so good, however after reading some sample code I wonder is it necesary to have a high level of test coverage? (I can see that most code is declarative and then add some attributes I can see the sense in checking so me particular attributes are there but not all)</p>

<p>Also i would be interested to know anything about what are the gotchas in TDDing with GWT</p>

<p>I m using eclipse so also if you are really happy with some particualrs add ins for GWT I would be happy to hear about that
Thanks for the input</p>

<p>edit: maybe I m asking a very wide question, but even little pieces of information will help
I come from having nvelocity views with jquery/extJs/prototype/scriptaculous and this is a bit different</p>

I think you asked a pretty broad question, which is part of the reason why you didn't get a reply for a while.
Compared to traditional AJAX web development, one could argue a GWT application requires less testing.  Because the GWT team has worked so hard to make sure that its widgets work consistently across all web browsers, you don't have to worry about cross-browser compatibility nearly as much for your own application.
That frees you up to focus on your own application.  Create a separate test case for each of your own custom widgets and test that they behave as you expect, and then write higher-level tests for each module.  Take the extra step to make your tests fully automatable - that way every time you make a change or are about to release, it's easy to run all of your tests.
http://code.google.com/docreader/#p=google-web-toolkit-doc-1-5&s=google-web-toolkit-doc-1-5&t=DevGuideJUnitIntegration