Does it, really? Most of the optimizations mentioned in the article (compression/obfuscation, module system, yslow integration) or in these comments (debugging, refactoring, auto completion) are all available/possible in javascript alone. What tangible benefits require Java's type system?
Guided code splitting and type tightening (with associated devirtualization) are serious optimizations that can't be done today in JS. GWT can guarantee the immutability of JS object shapes - something you can't do with bog-standard JS.