Have you tried Google Mobile app for iPhone. "You may encounter a problem using Google Calendar. In theory you can sync more than one calendar with iPhone using Google Sync. So, you go to m.google.com/sync in mobile Safari and set which calendars you wish to sync (by default only your "primary" calendar is included). So that's great except this particular page, for iPhone and iPad are being routed to dead-end pages (photo). There's a link for "How to setup Google Sync on iPhone" which refers you to go to m.google.com/sync. Causing a loop."
"The cause is that Google changed the URL but didn't document it." Here's the proper URL for adding calendars to sync with iPhone: https://www.google.com/calendar/iphoneselect