GWT is weighed down heavily by its mission to be both a Java->JS compiler and a UI framework. Scala.js is, first and foremost, a Scala->JS compilation mechanism, and only incidentally can have a type-safe interface to UI (or any other) pure-JS libraries.

It's less "heavy"; more concretely it's less of a parallel world like GWT. It's not tightly coupled to its own standard library like GWT, and it's more amenable to integrating with (javascript-)native libraries.