// add a button in GWT code
layout.addMember(toolbar);
layout.addMember(map);
// wait for the map to be loaded cause we need a correct map.getPixelPerUnit
map.getMapModel().addMapModelHandler(new MapModelHandler() {
public void onMapModelChange(MapModelEvent event) {
ScaleSelect scaleSelect = new ScaleSelect(map.getMapModel().getMapView(), map.getPixelPerUnit());
Double[] customScales = new Double[] { 1.0 / 100000000.0, 1.0 / 50000000.0, 1.0 / 2500000.0 };
scaleSelect.setScales(customScales);