There are a few options out there - I use xDrip (as the app to read my BG) - there is an Apple alternative to xDrip I believe but not sure what its called).
And then send it to an Android smartwatch (currently Google pixel 2, but have used fitbit Versa, and any range of Android watches). I'm also an old 'Pebble Smartwatch' fan (now out of production but still great) so keep those as a backup while my android watch charges.
Essentially you need a CGM and something to receive the signal on your phone, from there an app to send it to a smartwatch app.
Have a look at Dexcom and Apple - I think that works somehow but I'm an Android chap as can do more with it (looping etc)