While writing this I figured out a workaround but I’ll leave it here for future reference or if someone knows a cleaner way to do this.
Problem:
Want to route PC messages from external hardware to other external hardware without switching variations.
Setting “Accept Program Change Messages from selected devices only” to not include the sending device doesn’t work, then Gig Performer wont let through any PC messages from this device (Global Midi Monitor does not show incoming PC).
Workaround:
Set “Only accept program change messages on specific channel” to an unused channel. Then GP still receives PC messages on other channels but won’t consider them to switch variations.