No, there’s a limitless selection of alternatives that will work to control the gimbal/servo, and you can also set up a switch that changes control of the gimbal from various inputs. For example, a three-position switch can change control between the head tracker, a slider on the transmitter, and a constant value or Betaflight function like ‘cam-stab’.
What else would work?
Anything with a potentiometer, a slider, a guitar wah pedal, paladin sticks (pictured bellow), custom pivot control, the imaginations really the only limit. However, the control needs to be somewhat precise and able to be used dynamically while you are flying, and not be so unnatural that it stops flow state.
Why even use a head tracker?
I believe a head tracker is a great solution for this, the biggest limiting factor for it is that you can’t use it inside of a vehicle that’s constantly changing pitch, like a small boat, although I am planning on building a simple control unit that goes from shoulders to the goggles so that you can still use your head without a gyro, or even get fancy and let another head tracker to be fastened to my torso and be used as a reference angle.. but before getting too ahead of myself I’ll finish by saying that I see paladin sticks as a strong runner up to head tracking, although I wonder if twisting the sticks would affect control of the other channels a little, will have to try it and see.