Generally, I see Axon as a typical application concern, completely independent of the UI. GWT, on the other hand, is a UI concern, which should be completely independent of application logic.
I have built do (completely different) GWT applications using Axon. Both implementations use the GWT-provided servlet to implement services that are called from GWT code. On the server side, these services perform queries and/or send out commands. In some cases, we have uses a more generic “CommandService” which takes a command from the UI directly, and forwards it to the command bus.
So there is currently no Axon module for GWT. Although it sounds nice to make a seamless connection between UI code and the backend (by providing a commandbus and eventbus in the UI, for example), I doubt if that is an abstract that you overall application will benefit from. Clearly separating the UI from the application just sounds too much of a good idea to me