Does gwt have (good) support for effects, moving around elements, and resizing them? Are there any possible limitations GWT might bring to the table?
Yes you can use the Google Web Toolkit in Desktop. I use it with Adobe Air. After you compile GWT only generate JS files. But GWT in my point of view is very limited in effects.
If you want use web tecnologies in desktop why don't use Extjs is js framework with cool aspect and good libraries of effects and components. With Adobe Air is like an Desktop app.
The GWT API does not support all of these things natively. The newest versions of GWT have support for animation, however.
The useful thing about GWT is that it is an abstraction above javascript and it supports reuse through java in a much deeper way than javascript ever can.
Check this out if you're impressed with ExtJs. The showcase for gwt-ext should show the types of things that you can do with ExtJs but that GWT supports with the full power of Java. Of particular interest to you may be the portal and drag-and-drop examples. Setting up drop targets, dragging things around, and resizing things are where its at. The gwt-ext and ExtJs libraries support these things. It's up to you what level of abstraction you want to choose.
If the added overhead of all the ExtJs libraries are too much for you in gwt-ext then the gxt project from ExtJs may be more your speed. This is a pure java implementation of ExtJs and (as such) only the widgets you need are compiled. This reduces overhead but does not have all of the features that gwt-ext has.
Bottom line: GWT has powerful abstractions that you can build on to produce whatever you want. If you're not comfortable with building from scratch, you'll need another library.