Yeah, that's unfortunate. Not that another $5.99 subscription is bad, which I was gonna do anyway, but free is better.I think I tried that as I have a SXM sub through another car. Worked for a while and then it started barking about the trial was over and asking for a new subscription to be setup in the Gravity.
Even though it's really just a glorified cellular Android app there, it is behaving like the satellite radio model and wanting one subscription per vehicle.
But with Android Auto now, it isn't really needed since I get it there. The interface isn't as good for changing channels, but I may just stick with that anyway since my phone cell signal is better than the car's.
If the car had a real SXM antenna, I'd definitely get it's own subscription. But alas, another compromise.