I'm having a similar issue. My google calendar is working well on my iPhone. When I add new event in the calendar app in the iPhone, the iPhone automatically sync it to google calendar. I've been using the phone with this setting since ages and have never had any issue with that. However, now I've decided to forget my google account and have all my old present and future events on the iPhone calendar. I'm not planing to set up any email account on the iPhone anymore. But what I've noticed is that after I deleted all my accounts from the device, my google calendar show no event at all. I then set up back my google email/account on the iPhone, go to the calendar app and all my events are there.
My question now is, how do I manage to get my google calendar events out from the google calendar and have it sync to my iPhone calendar on a way that after I remove my google account, I could still have all my events present in the calendar app of the iPhone.