HIL with Gazebo Garden and custom model

Hi guys,
I am struggling to reduce the gap between Sim2World simulation using Gazebo Garden and PX4 1.15_dev. I have noticed that SITL is overly idealistic when tuning gains for an offboard controller in Gazebo. Even with a correctly modeled and parameterized simulation, there is a significant discrepancy between simulated and real-world conditions.

I want to understand if developing a custom controller in SIH can help me reduce this gap. I am wondering if anyone has tried using SIH with Gazebo Garden or Harmonic. Is it possible to use SIH with Gazebo Garden? Can I use a custom model with SIH? Where in the PX4-Autopilot codebase would I find the relevant code for this ?