Hej!
There is no widget provided directly in GWT, but there is a fantastic 3rd party widget that you can use that provides access to all the Google Map API to GWT.
I think the chap that wrote the widget also has this site -
http://www.gtraffic.info/ - (apologies to him if not!!) which shows the widget in action.
You can find the widget here:
http://sourceforge.net/projects/gwt/ and some instructions here:
http://groups.google.com/group/Google-Web-Toolkit/browse_thread/thread/7130539c37e8114/c7ae9a289867a66f?lnk=gst&q=Google+Maps+GWT+API Good luck with it - I've played with it quite a bit and it's very useful!!
//Adam