Try this way this may not really what you want but it may helps you, one of the best method that I follow many times. so through this you can debug also GWT modoule and server module both at a time.
It should probably a little bit easier, but it's possible. Here's how I do it:
1. Setting up the dynamic web project
File > New > Project... > Web/Dynamic Web Project
Select the target runtime which is tomcat.
Important: In Context Root, enter /
2. Adding the GWT module
Project > Properties > Google > Web Toolkit > Use Google Web Toolkit
Important: I always need to change the order in the Java Build Path
(Project > Properties > Java Build Path > Order and Export)
Project > Properties > Google > Web Application: War directory = "WebContent",
also check "Launch and deploy from this directory"
Copy your GWT contents to newly created project
run project as GWT application and also a dynamic web project.
Now, finally, you can debug both the server and the client part (I still need to click two Debug icons to start both!) I can manage the client side from the "Development Mode" view, and the server side from the "Servers" view. At the end of the day, it works great, and using the "Automatically publish when resources change" feature (Servers view > myServerName > Open > Publishing), sometimes even better than with the integrated Jetty.
Hope this will helps you.