Turbine engines are terrible engines for directly driving an automobile's wheels.

If you use a second shaft as a power-takeoff, they're actually not bad at all. You can lock the output shaft/turbine when the vehicle is stopped, and it behaves a lot like a torque converter with massive torque right off idle. GM used that exact method in the turbine semis, so I suspect it was the same for the Eco Jet. They used a simple gear reduction to get the engine speed down to normal engine speeds, and attached it to a normal old 6-speed.

I'm definitely not saying it's a perfect solution, I recognize there are a million other considerations for turbines in road vehicles. And that there's plenty of reasons why turbines never became commonplace for cars/trucks.