Q: Unable to turn off gmail notifications on Apple Watch

This exact same thing is happening to me as well, but with Facebook Messenger.

iPhone 7 plus / iOS 10.0.2

Apple Watch Series 1 / watchOS 3.0

Hi

The following steps should help:

• Toggle the notifications setting on and then off again, allowing a few moments in between.

• Close the Watch app and then restart both your iPhone and your watch:
  • To close the Watch app on your iPhone, double-click the Home button, then swipe up on the Watch app preview.
  • Turn both devices off together, then restart your iPhone first;
    • Restart your iPhone, iPad, or iPod touch - Apple Support
    • Restart your Apple Watch - Apple Support

• If the issue persists, it may help to unpair and re-pair your watch:
  • The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
  • Unpair your Apple Watch and iPhone - Apple Support
  • Set up your Apple Watch - Apple Support