BTW. to let you know.
You dont even need to use SDK to work with Engine.(even wiki say its suggested, but as optional you can use ANY IDE - gradle build is preferred)
For example, you dont need converts GLTF → J3O
You can just do:
Spatial model = assetManager.loadModel("Models/MyGLTFModel.gltf");
no need j3o 
But i think j3o is more “compressed” and should load much faster
but when you develop, until game is done, its easier to just load .gltf and skip j3o export part.