This took me forever to figure out. My wife shared a google calendar and I could see it in my Gmail account, but I could not view it on my iPhone. I had my wife share and unshare and re-share the calendar. On the iPhone tried going to Settings > Calendar > Accounts > Other and adding a “Subscribed calendar” and CalDAV Account using the URL from the email, but I keep getting an “Can’t connect using SSL” error. When I hit continue I’d get a “Unable to verify account information” error.
The solution was go to to this URL, https://calendar.google.com/calendar/u/0/syncselect and put a check-mark in the calendars that I wanted to show up on my iPhone. I killed and restarted the two calendar apps on my phone and the shared calendars showed up. This will probably also work if it’s your own Google calendar.