Maybe changing pose in gazebo model's sdf file will do the job? You can find it under Tools/sitl_gazebo/models/model_name/model_name.sdf
There is something like x y z roll pitch yaw
So changing yaw to pi/2 radians (you should calculate this value in radians and put it there) probably will do the job. Just change the top level pose because there are poses for each component of model as well everywhere below.
Also you can rotate your model in Gazebo GUI. Just click on it and choose rotation icon (at the top of the model's window 3rd in the row) then you can rotate it in 3 DoF.