The default behavior for Google Calendar synchronization is to grant access via CalDAV to all calendars stored within your Google account but not to shared calendars. To change this:
- Visit the following page: https://www.google.com/calendar/syncselect
- Log in with your Google account and select which calendars you want to synchronize than click Save.
- On your Windows Phone device, go to settings > email + accounts
- Locate your Google account, long tap and select Sync
- Once the sync completes you should have all the desired calendars visible in the main Calendar app.
Note that the instruction above apply for devices synchronizing via CalDAV. If you are a Google Apps for business user and have your account configured to synchronize via EAS you can still control which caledars to synchronize by visiting the following page from the web browser on your Windows Phone device: https://m.google.com/sync/settings/