You can display Google Calendars on your iOS device downloading the Google Calendar app or by using the built-in iOS Calendar app.
To use the iOS Calendar app that comes with your iPhone/iPad:
If you have a Google/Gmail account and want the calendars to show up on your iPhone or iPad Calendar app, you'll need to first make sure your Google account has been setup on your phone:
A. Go to Settings
B. Scroll to and tap 'Mail, Contacts, Calendars'
C. If you see your Google/Gmail account listed, skip to the next section.
If you don't see your Google/Gmail account listed:
I. tap 'Add Account'
II. Tap 'Google'
III. Enter your Gmail address and password, then 'Sign In'
IV. Tap 'Allow'
V. Make sure the options you want available on your iPhone are 'Green' (in this case, Calendars, but others if you want).
VI. Tap 'Save' in the upper-right.
Once you have your Google Account setup on your iPhone/iPad:
1. Go to the Upcoming Events page and click the '+Google Calendar' button on the lower-right of the calendar to add it to your Google Account.
2. Go to https://www.google.com/calendar/syncselect and log in using your Gmail address and password.
3. Put a checkbox by the calendars you want to see on your phone. Be sure to scroll all the way to the bottom of the list.
4. Tap the 'Save' button in the lower right of the window, at the bottom of the list.
You may need to tap 'Calendars' in the Calendar app and select the calendar to display on your device, like you would any other calendar.