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.
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. Please consider voting up for Garmint to fix it here.