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.
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
I suggest your can sync your account with
Calendars and change
Calendars notification sound.
Settings > Calendar > Accounts > Gmail(your account) > turn on sync Calendars