In order to launch the gazebo simulation, you have to source the px4_init file. You must also run the serial2udp.sh script in order to set up the bridge between the serial output from mavlink lora and gazebo. The launch file for mavlink lora also has to be set to point at the serial port /tmp/ttyMAVLINK

Then use one of the following commands to launch

roslaunch px4 posix_sitl.launch

or

roslaunch px4 posix_sitl.launch gui:=false