The output location for the GWT compiler differs when it is run through Maven vs. when it is run through the Google Eclipse plugin. What we want is for the Google Eclipse Plugin to target the same webapp directory as m2e-wtp and maven-gwt-plugin do.
Specific example of the issue:
1. Create a project from the Errai Kitchen Sink quickstart (you can create it with mvn archetype:generate -DarchetypeGroupId=org.jboss.errai.archetypes -DarchetypeArtifactId=kitchensink-quickstart -Dversion=2.0.Beta1)
2. From within Eclipse, run maven package on the project. Notice the GWT compiler writes its output to src/main/webapp/KitchenSink/**. This is good, because WTP is configured to use src/main/webapp.
3. At this point, you can run as web application on AS 7.1, and it will work. But this is GWT production mode, so changes made to client side code will not show up until the GWT compiler gets run again.
4. Try to launch Dev Mode (Run As -> Web Application Running on an External Server). Note the error message (the directory the GWT tooling was looking for doesn't exist) *This demonstrates the configuration problem this issue describes. GWT Dev Mode within Eclipse should be looking for the webapp in the same place as gwt-maven-plugin (and WTP)*
Note: there's a video of this entire process, including the workarounds, at http://www.youtube.com/watch?v=KaLjOjBPVp0