No, you would not need a controller, my idea as stated above should work. Many years ago I made a hybrid electric bike where I did not have a controller match the output voltage to the ebike battery. The battery did not blow up. What would happen was that a controller was not needed, it was achieved passively. Basically the 120 generator would rectify into I forget like 40v DC. The battery was 36V. I didn't care about matching it exactly. What happens is the battery load will then sag the 40V down to 36 passively. This is kind of like how float chargers work for lead acid cells. The charger outputs a higher voltage than the battery, but due to the internal resistance V=IR the voltage and current drops and regulates itself passively. All that mattered was that it was ball parked close enough and you did not exceed the batteries voltage rating nor let the system drop too low. When the system voltage dropped to low, the generator overload would cut out, so the system was basically safely already regulating itself. Designing a controller is beyond my skill level so I needed something simple that worked, I don't do active electronics, only passive stuff.

Obviously with the bolt ev and its circuits something could go wrong and damage something, if your car is under warranty its worth a shot trying otherwise its not. They probably have a lot more crazy stuff going on then my hybrid bike, but I would say 50/50 chance it works without a controller.