I don’t think there’s any watch that monitors BG directly. Yet.
I know Apple keep hinting at it for future iterations of their watch but it’s not a reality at the moment.
I wear an Apple Watch and self fund Libre 2. I’ve installed an app on my phone called Shuggah. That pulls the data from the sensor every ten minutes or so without needing to be scanned so it’s as close to as CGM as I can cobble together.
It works fine and Shuggah writes to the calendar app which can be displayed on the watch face. Sounds complex but it really isn’t.
Is it that sort of thing you’re thinking of?