Please Help! How do you add custom code to Navigation stack?

The motor controllers on my robot require GPIO actions for direction change. While I have the code in Python using Jetson.GPIO, I need to connect to the Isaac SDK, so that the SDK can control the motors correctly.

Which part of the SDK controls the direction of the robot?

The Isaac SDK seems to control motors with velocity, where +velocity is a direction (i.e. clockwise) and -velocity is opposite rotation. Therefore, you need to customize your code to set the GPIO pins accordingly when it gets a + or - velocity.

That is correct. The Isaac SDK navigation stack outputs trajectories of the robot in the world frame as sampled states each of which specify target position, velocity, and acceleration. A motor controller would need to interpret this trajectory into a series of motor currents per time step, for example.