I've run into a similar problem using GMaven 1.3, Groovy 1.7.5 and Maven 3.0. Generics in a Groovy class work ok so long as the parametric type is formally declared by a class, and not dynamically declared on the signature of a class definition. By this I mean that this is ok...
public interface MyInterface
public List<Person> getPeople();
...but this causes an error in the GMaven plugin at compile time...
public interface MyGenericInterface<Entity>
public List<Entity> getEntities();
This is the error occurs...
Failed to execute goal org.codehaus.gmaven:gmaven-plugin:1.3:generateStubs (default) on project gmaven-test: Unexpected node: Node[17:37,71,TYPE_PARAMETERS]
Is this a known issue?