We have just commited our new Leaflet jsinterop wrapper to Github. You can use Leaflet from your GWT app exactly in the same way as as you do from a javascript with slight advantage: static typing. The java code was partially auto-generated from Leaflet docs.

The project is still in experimental phase, and it will be improved based on the community feedback. For more info, you can check the Github page of the project: https://github.com/gwidgets/gwty-leaflet