You will definitely be very slow in building stuff initially, if you have not used GWT before. Much slower than using RoR. Unless maybe you've done lots of SWT/Swing programming, then you will at least have a general understanding on how to layout stuff graphically and use event listeners.
Everything in GWT is treated as AJAX app, in that all RPC's are asynchronous. But all GWT apps dont have to be "100% full screen". You have the freedom to do whatever you want visually, including wrapping JS libraries for animation, drag drop, etc (which I've done with my current project).
That being said, I agree with your comment about not everything fits the mold of an Ajax app. Like if you are not building a web application. Rails will be faster and more immediately gratifying in most situations.