No, you can't use GWT.
Now that GWT has been open sourced is there anyone looking in to integrating it in? I know you have done additional work to integrate a4j, and they have a tool to use GWT... It would certainly be a big step if GWT could be made a first class view technology for JSF.
There was an article of TSS about using GWT with JSF. (http://www.theserverside.com/tt/articles/article.tss?l=GWTandJSF) You might want to give it a try and see how it goes. It looks to me like it is too far removed from Seam contexts to give you the things you want from a Seam app, but it might have some uses.
Perhaps future JSF specs will provide GWT-like simplicity.