Please first create a backup using "Settings > Backups > Backup now > iCloud" as a security precaution.
Then you can try to re-download all calendars (but not their names and configuration) using "Settings > Debug Tools > Reset all calendars". The download will probably take at least a few minutes.
If that doesn't fix the problem, please try to remove and re-add your Google account. Open "Settings > Accounts" and swipe your Google account to the left and select "Delete" to remove it. Then tap "Add account" and add it again.
Does that work? Please try this and don't hesitate to contact me again if you still have problems or if you have any questions!
Kind regards
Dominique Gunia