After changing an "Activity or Calendar Events Info" to Google Calendar in the watch settings:
(Optionally) You can add more calendars by entering their ids (typically a mail). Paste them from Google Calendar settings to Garmin mobile Connect settings of the watch separated by space or comma. Learn more about the calendar settings and behavior.
Change "Activity or Calendar Events Info" in Garmin mobile Connect app watch face settings to "Google Calendar".
– If you don't see such option there, you watch does not support it.
Get a link and code from your watch
When succeeded, wait for the watch to log-in. It can take 5 minutes or so.
The watch will load your primary Google calendar. You can change which calendars are shown in the watch settings.
– If you enabled a weather forecast as well, you'll need to subscribe for it before the calendars are shown.
If the calendar did not load despite you logged-in correctly, you are unfortunately among a fraction of users whose device experiences a known Garmin bug. Typically this bug keeps showing new and new login codes indefinitely. Please consider voting up for Garmint to fix it here.