I'm no expert but I think Clojurescript takes the Clojure language (spelt with a "j") and converts it to unoptimized Javascript, which then gets processed by the Closure compiler (spelt with an "s"). The Closure compiler takes unoptimized Javascript and emits optimized/minified Javascript.
Closure is from Google, but is independent of GWT which is also from Google, despite the two projects having similar aims. Closure is modern, whereas GWT is not.
The open source team who maintain GWT now (after Google abandoned it) are in the middle of a re-write, which will be called GWT 3 (current version is GWT 2). GWT 3 will convert Java to unoptimized Javascript and use the Closure compiler to optimize it etc (as Clojurescript does). Current GWT 2 does not use Closure but does its optimizations itself.
I may have got some of that wrong, corrections welcome!
They generated a Closure compiler implemented in js by running the closure compiler through GWT, but it's worse than the java equivalent (fewer features, slower)
