See this link to use the Gazebo classic simulation.
The repo have been modified to use RealSense camera with the Iris model. After the setup in the above link, you may use
roslaunch px4 px4_mavros_D435i.launch
to launch the simulation.
A Livox Mid360 model has been added to this repo for lidar simulation as well. To use the Iris model with Livox Mid360 lidar simulation, you may use
roslaunch px4 px4_mavros_livox.launch
to launch the simulation.