piotr-j, AKA EvilEntity, is working on a graphical editor for gdx-ai behavior trees. It's still a work in progress though, so don't expect too much.
However, I don't think that such an editor is crucial due to the power and simplicity of the DSL. Especially, gdx-ai-1.7.1-SNAPSHOT adds internal subtree references, guards, and dynamic guard selectors that allow you to write complex trees while keeping them pretty human-readable. Here is a sample tree. Unfortunately, the documentation for these new features is still missing, but I'm going to write it soon.
Anyways, if you're happy with JBT there's no real reason to use gdx-ai btrees unless your game has to run on GWT (javascript is single-threaded) or mobile platforms (having a lot of threads might be a serious problem, especially on old devices).