But...that's not possible in the first place, is it? Or was this back before they dropped support for that? Or were you using Google Apps?

The most success I've had with syncing calendars between platforms is with Outlook and GMail using the gsyncit tool. It's not free, but it works pretty well. I installed it for a client who checked her standard GMail account in Outlook and wanted her contacts and calendar to synchronize. It works pretty well, but she had to upgrade the tool and pay for a new version when her computer forced her to upgrade to Windows 10.