No JS library like jQuery (especially jQuery) makes sense. jQuery is useful, but the code written with it tends to be fragile (and when things blow up because of an IE6 bug in jQuery - and they do happen - you're pretty much SOL)..
In fact, the only kind of library I could consciously condone is one that promotes object oriented design by not giving you so many easy (but SLOW) ways to access DOM elements (I'm looking at you, $()).
Of course, it all depends on what you're trying to do. GWT tends to ease the data transfer a bit, from what I know, plus being easier (I assume) to integrate with Gears. So if you're doing a lot of data manipulation and storage with AJAX, GWT may actually be the way to go.
If you feel comfortable writing JavaScript, that's probably the direction you want to take. GWT seems like a platform for Swing programmers who don't want to deal with JavaScript's idiosyncrasies and browser quirks.
But for all I know, I could just be blowing smoke - I've not delved that far into GWT. Seemed like too much trouble for what you got from it...
