After you changed an "Activity info" to Google Calendar in the watch settings, keep the watch connected and:

  1. Enter the code from the watch screen to and log-in
  2. Wait at least 5 minutes until the watch loads your primary calendar automatically

(Optionally) You can add more calendars by entering their ids (typically a mail). 

To get more calendar ids paste them from Google Calendar settings to watch settings separated by spaces

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