Is it legal to use Garmin icons for watchfaces?

I doubt you'll be able to get a legal answer here, even if a Garmin employee (who usually are technical, not legal) sees your question. If I were you I would use the icon until/unless Garmin will indicate there's any problem with it. If your app is approved, it should be ok, and even if in the future they'll "crack down" on apps that use such icons, it won't be hard to change the icon