You have to refer to the jQuery object $
as $wnd.$
.
When accessing the browser's window and document objects from JSNI, you must reference them as $wnd and $doc, respectively. Your compiled script runs in a nested frame, and $wnd and $doc are automatically initialized to correctly refer to the host page's window and document.
http://www.gwtproject.org/doc/latest/DevGuideCodingBasicsJSNI.html