Hi V_boyd,
It sounds like you have your Google calendar syncing to your iPhone. You can check by tapping Settings > Calendar > Accounts > [Gmail Account]. From there you'll want to toggle "Calendars" off if you do not wish to sync the account to your iPhone.
You mentioned your Apple calendar is syncing to your Google account. It sounds like you may have that account set as your default account:
Set a default calendar
You can set one of your calendars as the default calendar. When you add an event using Siri or other apps, it’s added to your default calendar.
1. Go to Settings
> Calendar > Default Calendar.
Select the calendar you want to use as your default calendar
Set up multiple calendars on iPhone - Apple Support
If this is the case, calendar events that you add on your iOS device would sync to your Google calendar. If you don't want this to happen, you'd need to select a different default.
We hope that helps.
Take care.