Simulating an actuator gear efficiency & back-drive torque

I want to simulate a robot with a non back-drivable revolute actuator (i.e with a high gear ratio).
From my experience with IsaacGym, any DOF is back-drivable and couldn’t resist an external torque without applying an inner drive torque.
How exactly the gear efficiency is simulated? (I couldn’t find any reference to the gear issue in the documentation)