Make sure you inherit from gwt-openlayers by adding the following to your module file:

<inherits name= org.gwtopenmaps.openlayers.OpenLayers />

Also make sure you bring in the OpenLayers javascript library and OpenStreetMap OpenLayers into your application by adding the following lines in "Application.html" page:

<script src="http://www.openlayers.org/api/OpenLayers.js"></script>

<script src="http://www.openstreetmap.org/openlayers/OpenStreetMap.js"></script>

Then it should a simple thing to create a layer that uses open street maps:

OSM openStreetMap = OSM.Osmarender("Base Map"); openStreetMap.setIsBaseLayer(true);

MapWidget mapWidget = new MapWidget("350px", "350px"); mapWidget.getMap().addLayer(openStreetMap);

GWT-OpenLayersHelloWorldwith OpenStreetMap

The following example works fine for me usingOpenLayers-2.8/OpenLayers.jsandOpenStreetMap.jsas described above:

public void onModuleLoad() {

MapOptions defaultMapOptions = new MapOptions();
MapWidget mapWidget = new MapWidget("800px", "600px", defaultMapOptions);

OSM osm_1 = OSM.Osmarender("Osmarender");   // Label for menu  LayerSwitcher 
    osm_1.setIsBaseLayer(true);

OSM osm_2 = OSM.Mapnik("Mapnik");   // Label for menu  LayerSwitcher 
    osm_2.setIsBaseLayer(true);

OSM osm_3 = OSM.CycleMap("CycleMap"); 
    osm_3.setIsBaseLayer(true);

OSM osm_4 = OSM.Maplint("Maplint"); 
    osm_4.setIsBaseLayer(true);

Map map = mapWidget.getMap();
    map.addLayer(osm_1);
    map.addLayer(osm_2);
    map.addLayer(osm_3);
    map.addLayer(osm_4);
    map.addControl(new LayerSwitcher());
    map.addControl(new MousePosition());

 // map.setCenter(new LonLat(6.95, 50.94), 12);            // Warning:  In the case of OSM-Layers the method  setCenter()  uses Gauss-Krueger coordinates,
                                                           //           thus we have to transform normal latitude/longitude values into this projection first:
    LonLat lonLat = new LonLat(6.95, 50.94);               //           (6.95, 50.94)  -->  (773670.4, 6610687.2)
           lonLat.transform("EPSG:4326", "EPSG:900913");   //           
    map.setCenter(lonLat, 12);                             //           see  http://docs.openlayers.org/library/spherical_mercator.html

RootPanel.get().add(mapWidget); }

you find the current version of gwt-openlayers here: http://sourceforge.net/projects/gwt-openlayers/