Li Haoyi (scala.js author) explains why he worked on scala.js pretty well: http://www.lihaoyi.com/post/FromfirstprinciplesWhyIbetonScal...

Using scala.js with jquery-ui or any other js library using facades works well. If you wanted pre-made ui widgets, your choices are going to limit you in one way or another (compared with writing html/css), so arguably learning jetty+scala.js+jqueryui, is equivalent to using GWT, and has surpassed GWT in terms of js/scala/jvm compatibility in every way. It might never compete with webassembly in terms of raw performance, but then GWT isn't about to do that either if you're writing code for the browser.

Also, Scala+GWT seems to be dead, probably because of scala.js. You can use gradle/maven with Scala just fine. I use gradle for all of my Scala projects.