If you want to sync with Google Calendar, please use the following method.
1. Show Google Calendar events in iPhone Calendar
Please refer to this page.
2. Select Google Calendar as sync destination with this app's iOS calendar sync function.
2.1 Open Settings
2.2 Select iOS Calendar Sync
2.3 Select Calendar name
3.3 Chose the calendar that you want to sync.
The Google account calendar added in step 1 is also displayed in the list.
Under your Google calendar name, you will see your Google account email address.
Select the calendar you want to sync.