You can compare how the sdf files look like as trying out the instructions here without using ROS
The sdf files generated will be stored in /tmp/<vehicle_name>.sdf basically you need to configure the mavlink interface plugin : https://github.com/PX4/sitl_gazebo/blob/master/models/typhoon_h480/typhoon_h480.sdf#L1351 to be able to interface with multiple vehicles