Types can either be known (declared or inferred) or unknown, in which case IDEA marks them underlined. That is great!
Some refactorings like renaming and such can lead to the situation that a reference with formerly known type now becomes to be of unknown type. This is most often (but not always) an unwanted effect. It would be great if JetGroovy would raise a warning in such a case.
Additionally on my christmas whishlist would be a shortcut (F2?) to quickly cycle through all references of unknown type in the current file - to check whether there are any that should actually be known but are unknown because of typos.
keep up the great work