There are other mapping toolkits for GWT, like gwt-openlayers and gwt-esri, but I wanted something more lightweight and more modern, which also supports mobile devices. One project which stood out of the "mapping-jungle" was Leaflet,
So I went to it, and made gwt-leaflet, a first class citizen mapping toolkit for GWT. It wraps Leaflet using JSNI and Type Overlays, which expose Leaflet API as Java interfaces and classes. It is still early days, and some work remains before all relevant parts of Leaflet API are exposed in GWT. For the technical inclined readers, I have prepared a short summary at github, which goes a bit further in explaining the technical aspects of the project. The nice map below is the example project running as a Google app at appspot.com.