GwtQuery a.k.a. GQuery is a jQuery-like API written in GWT, which allows GWT to be used in progressive enhancement scenarios where perhaps GWT widgets are too heavyweight. It can also be used to find and improve your GWT widgets.
GwtQuery is easy to learn for those using jQuery as they share the same api, aditionally gquery adds nice features like type-safe css, compile time optimisations, etc.
Currently, almost the jQuery API is written and all CSS3 selectors should be supported.
For more information/documentation : http://www.gquery.org