Configure Google Calendar notification sound on iOS On iPhone we can configure notification sounds for iCloud Calendar and the iPhone Reminders app, but I cannot find any way to configure a sound for Google Calendar app on iPhone. I find the default sound to be too meek.
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