1) Using a desktop browser (just easier), go to your Google calendar.
2) Click on the calendar you want to sync and go to "Calendar Settings"
3) Scroll to "Calendar Address." You will see colorful link buttons labeled "XML," "ICAL," and "HTML."
4) Click on "ICAL" and email the address that pops up to yourself
5) Click on that web address using your iPhone
6) You will receive a pop-up that asks you if you want to subscribe to this calendar. Click yes.

Your calendar should now pop up.