Configure Google Calendar notification sound on iOS
Be reminder.

There is no way to customise the notification sound for 3rd party applications until the app developer build that functionality into their app. Applications like WhatsApp has options for that, you can change the notification sound from the app settings itself.

I suggest your can sync your account with Calendars and change Calendars notification sound.

Settings > Calendar > Accounts > Gmail(your account) > turn on sync Calendars