Okay, the problem seems to be in the GR2 file itself, and not in the converter. As you can see, the original RO GR2 file holds several meshes, but just one model, while the other holds the same number of meshes and models. This means the game will read only the first model of the GR2 file, which, in this case, is just composed of the first mesh. I'm not quite sure how to solve this, but it's something that has to be fixed at export time (when you export using 3DS Max).
EDIT: Found a solution, and I already told Olrox on Skype about it. Just group the meshes when exporting and they will be considered a single model by the granny exporter.