Hello Jim,
There is a feature request for something similar at:
https://www.playitsoftware.com/Support/FeatureRequests/4/Remote-Deck-Start--Serial-COM-port-
You will see from that MIDI is now supported but not GPI relays.
https://www.playitsoftware.com/Plugins/View/RemoteStart
I would like to see that feature extended to include GPI & GPO.
Depending on what you are trying to achieve there may be other ways with PlayIt Live but if you are trying to get local playout initiated and stopped by a control signal from a "sustaining" feed, there isn't an obvious way to do that.
PlayIt Live will play streams from remote sources (via the aux input feature if needed but natively as a stream as well) but needs to be the "timing master" for doing so with its scheduled events.
Jim Craven