It seems that licensing terms would prevent us from using Google Maps API in our software. We would prefer to be absolutely free open source and Google Maps API will require commercial license if our software is sold (correct me if I am wrong - I will delete this question then :-).
So the question is if there are alternative map APIs for GWT?
Openlayers is free software, http://openlayers.org/; it has a GWT plugin here: http://sourceforge.net/projects/gwt-openlayers/
As far as Google's License (http://code.google.com/apis/maps/terms.html):
10.9 use the Service or Content with any products, systems, or applications for or in connection with:
(a) real time navigation or route guidance, including but not limited to turn-by-turn route guidance that is synchronized to the position of a user's sensor-enabled device; (b) any systems or functions for automatic or autonomous control of vehicle behavior; or (c) dispatch, fleet management, business asset tracking, or similar enterprise applications (the Google Maps APIs can be used to track assets (such as cars, buses or other vehicles) as long as the tracking application is made available to the public without charge. For example, you may offer a free, public Maps API Implementation that displays real-time public transit or other transportation status information. If your Maps API Implementation is deployed internally or you are charging for use of your Maps API Implementation, please contact the Google Maps API Premier sales team for more information);
My advice would be to contact Google and ask them about your specific set of circumstances. I am not a lawyer and this is not legal advice.
If you are looking for full open data you might want to check out OpenStreetMap.org they list in their FAQ where they pull their open data from.
I'm not an expert, but a quick google found GWT-Ext, where it looks like they're using OpenStreetMap data which is free to use in a commercial application.