Hi Marco,
I've just received the same response from Libellium. I didn't find that in any of the documentation at all. Which is a bit of pain given the amount of time I've spent researching this. It's also a shame because the Gateway software looked very capable.
I just don't understand why would adopt a standard then run your won proprietary protocol on it.
So I'm going with the Multitech Gateway. I think that is decision made.