Yes, a gateway is a common solution. Or having two busses (one for engine control and the second for aux functions like HVAC, radio, lighting etc) with a system to pass critical messages between the two.

But it's cheaper to use a single bus and just slap everything on there. Or in the case of something like OnStar, realize you can add extra capabilities through firmware and not fully think about the impact when your radio can send unfiltered messages to your ECM.