GWT brings all the Java IDE goodness to what is essentially JavaScript development. Like JQuery, GWT "normalizes" the browsers for you, so you typically don't have to user-agent checks or anything. What the Java IDE goodness gets you is really reliable refactoring support for changing Names, Packaging, Method Signatures, all kinds of stuff. ITS BEEN 2 YEARS SINCE I'VE DONE GWT, but I think its downside, especially on large projects, is the compile step. It is not fast, especially when you let it loose to compile all the Java for each of the browsers. If I recall correctly, and if it still does this today, it produces on really big JavaScript file for each of IE, FireFox, Chrome, Opera, and Safari. That takes time, my friend. There is a unit testing framework in there as well which is interesting, albeit slower than its pure Java counterpart.
GWT Benefit: Robust Tool support via Java IDE ecosystem.
GWT Liability: Compile time
I don't know nearly as much about JQuery other than it too seems to have cracked the browser compatibility cookie allowing you, fair developer, to concentrate on your application. What I do know about JQuery is that it seems to have developer mindshare. I, who am no JavaScript expert, managed to integrate JQuery into my Java webapp and add an image preview "plug-in" called imgPreview (http://james.padolsey.com/javascript/new-jquery-plugin-imgpreview/) giving pop-up image previews to my bland table-based UI in about 30 minutes one afternoon. The browsers have grown up too in the last 2 years such that you can now dynamically debug your JavaScript in realtime in the console (Chrome rocks for this, but it is essentially the same in FireFox and Safari), as well as tweak the styles and do all kind of straight DOM manipulation.
JQuery benefit: Straight up JavaScript with no interpretation and developer mindshare
JQuery liability: It is challenging to refactor JavaScript at anywhere near the level of a Java application.
My 2 cents worth.