Replacing Flute with Closure Stylesheets while retaining GWT's semantics is not easy. First, Flute is a SAC parser while Closure is DOM-like using visitors (much like GWT's own internal model).
Given Ray's earlier idea to even move to SASS or LESS, I'd rather introduce a new GssRessource (which can even be done as a third party lib), and possibly merge them later if possible. -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
