It is currently possible to add a Garmin Connect Calendar to a Google Calendar by adding it via its URL. However, there are two problems:
1) The calendars do not seem to be immediately synced: if you add something from within the Garmin Connect Calendar application, it is not immediately reflected in for example Google Calendar. It probably takes a day or so to sync.
2) Only the next 30 days are visible in Google Calendar. See also https://support.garmin.com/faqSearch/en-US/faq/content/UVAoDoRAgl1d75WQc4aGEA that seems to explain this.
The above two things are quite annoying for people who would like to see their Garmin Connect Calendar next to their other calendars for day-to-day planning.
I'm not sure whether this is a problem on the Garmin side or on the Google side, but I think 2) is the most important issue to solve.