gwt-esri
gwt-esri is a GWT JSNI wrapper around the ESRI JS API. It allows you to fully integrate with the ESRI JS library in GWT. This can be very useful when building complex online GIS applications as you can use JAVA to build the app.
Not to dive into a language argument, but there is something to be said about strongly typed, OO languages like JAVA over JavaScript. I think the real interesting aspect is how GWT, a compile to JS language, sort of went out of style for a bit. But now seems to be having a huge come back. Between ES6/ES2015+ transpilers, Grunt/Gulp concat-minify-uglify or strait compile to languages like CoffeeScript, TypeScript and JSX the world seems to have slowly swung back into favor of compile to JS.
I'm still waiting to see if GWT 3.0 ever makes it out the door. Supposedly Google Inbox uses GWT as well as other in house Google products. Guess time will tell.
For now, if you are looking for some GIS goodness in your GWT webapp. Look no further. ESRI's ArcGIS REST interface isn't half bad (if you can ever get the service setup and running properly). And their JS API adds a decent layer over the top of these services as long as you throw their 'UI widgets' out the door.