Kerstok is watch app that runs on most Garmin watches that allow third party apps. Since 1.6.0 also on watches without touchscreen. 
It can be installed from: https://apps.garmin.com/en-US/apps/b6348ccc-86d8-4780-8013-d9e19fed5260
It should be used with Juggluco, https://www.juggluco.nl/Juggluco/index.html

With it, you can enter numbers under a certain label on your watch. These numbers are sent to Juggluco.
If you have a Sibionics GS1, Dexcom G7/ONE+ or Freestyle Libre 2,2+,3 or 3+ sensor in your arm, 
Kerfstok can also display your current glucose value on your watch. The sensor sends via Bluetooth the glucose level to 
Juggluco running on Android and Juggluco sends this to Kerfstok on a Garmin watch also via Bluetooth.

I published the source of Kerfstok so people can modify it to fit their taste.

For more information about Juggluco visit:

https://www.juggluco.nl/Juggluco/index.html

For more information about Kerfstok, see:

https://www.juggluco.nl/Kerfstok/index.html

For information about how to build a Garmin watch app, see:
https://developer.garmin.com/connect-iq/connect-iq-basics/getting-started/#gettingstarted

To build the app you need to turn off type checkings: 
- When using Eclipse, you have to put  -l 0
in Windows->Preferences->ConnectIQ->Compiler->"Compiler time options". See: 
https://forums.garmin.com/developer/connect-iq/f/discussion/314861/sdk-4-1-6-generating-new-errors-and-warnings
- In Visual Studio Code, you have to go to the Extensions (View->Extensions), Press on the cog symbol beside Monkey, select "Extension settings" and set "Type Check Level" to off. 

Jaap Korthals Altes