Max, the gwt-maven-plugin's generateAsync goal generates code into target/generated-sources/gwt.
At the moment (3.3.0.Beta3) JBT does not handle the lifecycle mapping for this goal (and nobody else does, I thought the whole GWT-eclipse-maven integration would be a responsibility of Google's, but apparently they don't think so...). In fact, you have to add the lifecycle-mapping pseudo-plugin in your pom.xml to at least avoid error messages from m2e.
The GWT-Eclipse plugin is able to generate sources, but is not aware of the correct location because it is maven-specific.
The optimal solution would be to add target/generated-sources/gwt to the build path, and configure the GWT-Eclipse plugin to generate the classes there.
But I think this is another issue/thread, right?