Should you not be able to connect your iCloud calendar, you might not find it such a bad idea to consider moving your calendars (and even contacts) to another online service, which plays nicer with standard protocols, given that calendars are easily exportable and not necessarily tied to the email address.
I also am both a macOS and GNOME (via Fedora Workstation) user, and early on I realized it is best to avoid trying to integrate iCloud services into GNOME, and rather search for alternatives. There are several ones (both open-source and proprietary) which integrate well into GNOME Online Accounts.