Aren't they kind of, well, both? They're clearly a corporation, headquartered in Kusari. At the same time though, they operate primarily outside of House territory so they can be seen as a nation, too.
So... they're a corporation who have acquired a good deal of sovereignty based on the fact no one else is there.
What if Kusari had decided to move into the Sigmas instead of the Taus with force? I think the GMG would be forced to stand down lest they forfeit their primary corporate ally and their ability to supply H-Fuel.