This sounds like a bit of a train wreck. Anyone have a good link with an example of a third party usage to get things ultimately back to Google Calendar?
Is there something that isn't working? I checked now and I can see my planned workouts in Google Calendar.
More info: https://support.garmin.com/en-GB/?faq=UVAoDoRAgl1d75WQc4aGEA
I don't know who plans their workout schedule in Garmin .. that seems like and edge case / unusual feature. But as a new Garmin smartwatch user I would have assumed the primary use case for calendar integration or basic exporting is just show the record of the activities you log. There doesn't even seem to be a way to export Activities and manually import to Google Calendar
If you follow one of Garmin's training plans (https://connect.garmin.com/modern/training-plan) your training plan will be in the calendar. There are also several 3rd party services that can push training plans to Garmin. They will also be visible in the calendar. So, that is probably how most people use that function.
I don't know about any function to export your training history from Connect to Google Calendar.