Tutorial for the setup of ROS Gazebo SITL for Ardupilot
It contains automation scripts and a tutorial to help follow the instructions from:
http://dev.ardupilot.com/wiki/using-rosgazebo-simulator-with-sitl/
The whole setup process is described in "Tutorial to setup SITL Gazebo on Windows - vX.X.pdf". It is intended for Windows users, for it starts with the setup of a virtual machine on VMWare. Still, if you already are on Linux the 2nd part will interest you.
The scripts are located in the 'scripts' folder. To run them, do not forget to run "chmod +x ./*.sh" in their directory, to authorize their execution.
Hopefully it will help you setup your simulation environment !