There are no new options for this in FM GO 13. You can lock your entire phone into portrait orientation as that's an iOS option you can select, but you can't lock the FM GO application into either orientation.
What FM GO 13 does offer is a new script trigger that makes it easier to run a script when the orientation changes that allows your FM GO interface to better adapt to the current orientation.
In addition to locking the screen as Phil mentioned you can lock the screen from pinch zooms as well..
JP, Phil. Thank you both for taking the time to reply. Barrie.