Issues syncing gmail calendar to iCloud calendar

All the devices sync with an email client server, which hosts your calendar. If you have a Calendar on iCloud that will not sync with Google Calendar. Settings > Calendar > Default Calendar [Choose what you want]. Vice-versa is true.

What you add (Event) on iPhone Calendar is synced with the default calendar unless you have chosen a specific calendar at the time of event creation.

To which email account are the calendar events synced with your iPhone? This is not clear in your post

See the iPhone and Mac Calendar Pics below

Which is your default calendar? Settings > Calendar > Default Calendar

Settings > Calendar > Sync > [All Events] - Watch the GIF below

It's not always the iOS update's fault!  Many things can cause phone problems.  If you tell people you think it's an iOS update, it might make it harder to fix or find a solution for the real issue.

Remember, the right question often leads to the solution!

Was/Is your device Jailbroken? If that's the situation, please be informed that attributing the issues to iOS updates may not be accurate.

iOS updates aim to enhance user experience, boost security, and fix bugs rather than disrupt core features. The majority of the issues after updates might stem from underlying, unmentioned factors. Updates won't typically alter personal data but may refine settings or features. Backup before major updates for safety. Review Apple's update details and backup advice for a smooth process.

Kindly consider checking your settings and addressing any device issues before attributing the issue to external factors. This could be a pure coincidence that the iOS update affected the iPhone functions.

Before initiating a standard iOS update, it's advisable to create a backup. Have you completed this step? If yes, proceed with the restoration from the backup.

See what each update does: OS — iOS 26, About iOS 18 Updates, About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...