kaleksandrov, thank you for posing this question - I couldn't agree more how useful this would be - the GC (Garmin Connect) calendar has all these automatically produced ACTIVITIES & workouts from when…
If he/she then wants to plan training workouts in the lead-up to the race, again it could be useful to add those to one's calendar that also shows other (e.g. work, social) engagements for planning purposes…
Even the non-athlete who uses a Garmin device to track his/her sport activities can use Garmin Connect and/or Strava; there is no inherent barrier.
Last time i looked into this Problem is now 8 (!) years ago.
Well, if nobody used the form at Submitting an Idea to Garmin for suggesting it to Garmin, then it is not likely to change in the next 8 years either. Only if there is a high demand for such functionality, it has any chance to get implemented.
BTW, I did not test it, but the document Publishing the Calendar or Connecting with Office 365 in Garmin Connect tells the following:
« Activities, workouts, and events from the past 30 days will be synced to Office 365 once you are connected. After connecting, any new activities, workouts, and events will continue to sync to Office 365. »
It means, you can use the Office 365 Calendar and export it to your Google Calendar from there. You can also try temporarily setting the date on your PC or phone to a past date, to see whether it then exports also Activities older than 30 days. This trick works with some other features of GC, so there is a chance it could help here too.