You are on OSX which has a built in ability to create virtual MIDI ports called IAC (Inter Application Connection).
You can create a new port just for this purpose that will always be there. Simply open Audio MIDI Setup application on your Mac, then click on Window->Show MIDI Studio where you’ll find your IAC Driver. Double click on it.
You can create a special port just for this called GP->MS for example:
Now this port becomes instantly available to GP as any other MIDI out device:
This also becomes available as an input port in MainStage:
So no need to really have any third party applications or complicated setups. Once you’ve done this it’s always there like a permanently connected MIDI device.
Hope this helps.