After you have setup you iOS device using the following QuickAnswer Article: How to sync Google Calendar, Contacts, and Email to your iOS device (iPhone and iPad) - [Not Supported by ISS]
Although ISS does not "officially" support the Google Sync service and your iOS (iPad & iPhone), we recognize the utility of being able to take advantage of the tool. Therefore, we provide the following documentation as a reference for you to configure Google Sync on your personal device.
By default only your primary calendar will be synced to your device. You can sync additional calendars by visiting the following page from any web browser:
Select the calendars you'd like to sync, then click Save. The selected calendars will display on your device at the time of the next sync.
Reference: Google Sync Help - Sync Multiple Calendars