The slot for these appears to not do anything. The gun pod is always visible on the external model no matter what in the eden editor, and the undertail doesn't ever appear.

Furthermore, you can't equip a gun pod. I don't know if this is realistic but I would prefer some functionality for gun pods on the GR9. If you won't add the harrier gun pod, may I request we take a page from Project Wingman and just let us equip the vanilla gun pod on pylons?