You can globally listen for all browser events by using Event.addNativePreviewHandler(). Inside the handler you can globally check for shortcut keys on keydown / keyup events.
But be aware that there are inconsistencies between browsers for the return values of NativeEvent.getKeyCode() and NativeEvent.getCharCode() for keyup/keydown/keypress events. GWT does not handle these inconsistencies between browsers because it seems to be quite complex to do it right.
-- J.