There's metronome apps I've used for practice, Pulse i think it was called, makes the watch vibrate in time, but wasn't a strong enough vibration to really be useful for me. As for gig-useful, not sure, maybe could do something cool with a glowing light changing as you move?Watches would seem like a non-issue for playing, rings however…
I wonder if there’s anything gig-useful that you could use a smart watch for.