OK, this got resolved, however how, that I’m not able to tell 
But just to close the thread to a conclusion I’ll let all know what happened.
As said earlier, I took the gig file from my gig PC, uploaded it to my backup PC, and the metronome worked just as expected. I then did some work on the gig file with my backup PC, stuff that was totally unrelated to the metronome (added some songs to a setlist etc). I then took the file back to my gig PC, and voila - the metronome worked!
This is obviously totally unrelated to GP, so must have been something going on with either my gig PC or the physical setup of my gig hardware. However, all working fine now and GP is absolutely fantastic!