GWT is quite interesting in a sense that you can use on client-side regardless of the backend technology. So comparing it directly to Ruby on Rails isn't much fair because ROR is a full-stack solution (except javascript libraries for UI work). Still, in your case, if you want to use GWT, you should be clear with the following points:
- You'r comfortable with Java
- You app is purely web 2.0 (means no more than different pages than can be viewed as different modules)
- Forum, mail server integration won't pose any difficulties
- You have a bit more time for development
I would suggest that if any of the above points don't work out for you, then you should move to ROR. Now why I built a checklist based on Java? Because you'll find quick development tools like "Spring ROO"(yes, Spring Roo is the smartest code generator) and scalable, integrable, full-stack technologies like "Spring" in Java. And Spring supports the development of everything you've mentioned. And even if you think GWT won't suffice your needs(in terms of short development time) then you can easily go for JQuery and it's Spring glue library to develop UI stuff.
P.S: Spring Roo 1.1.0.M2 and GWT 2.1 have been integrated to reduce development time many folds but it's not production ready yet.