GWT is a compiler (Java to JavaScript) and jQuery is a framework. You don't have to choose one over the other. You can use one of them, both or neither.
For instance you can code in Java if you want to or have existing source code and then use jQuery for other stuff. There are available wrappers, but GWT can call JavaScript (and vice versa) see http://svenbuschbeck.net/wordpress/2012/06/how-to-use-jquery-in-gwt/
