I’ve never used AntennaTracker, but it appears that you’ll need an extra device to translate the PWM signals from the controller to a voltage output for the DC gear motor. The ONOFF mode still needs PWM, so it doesn’t really help you much here. My suspicion is that one of these motor controllers from Pololu would work really well.
You would configure the SERVO_PITCH_TYPE and SERVO_YAW_TYPE to ContinuousRotation, and the Pololu controller would translate the RC PWM signals to voltages to drive the two axes.
There are likely other units out there for ground robots that perform a similar function, possibly less expensive, but Pololu is usually an excellent source.