While GWT is purely client-side library and server side can be implemented in any programming language, it makes sense to use Java for server-side as well. There are a lot of tool, frameworks and libraries for web development in Java, and the fact that client side and server side are in the same language allows certain amount of code sharing, with restrictions, of course.
I have also discovered an interesting new software development language - Scala. It is a functional/OO language, geared towards Java developers, i.e. offering interoperability with Java and offering somewhat familiar syntax and, most important, compiling into Java bytecode (a .NET port is in development as well). This is great news, since using specific features of Scala helps solving some of the common problems of web development, without requiring to switch to another platform completely. One can see, that JVM is finally catching up with CLI/.NET in the actual availability of multiple development languages (some languages targeting JVM have existed for years, but none of them before Scala have acquired any considerable following).