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.