First off, I know this is quite old....
I've been trying to find an answer to this and couldn't find anything. However, the above answer is not what you are looking for (I think).
What you need to do is open up Setting on your iPhone scroll down to find Google's calendar app. Click on that and turn the "Allow Full Access" toggle to on.
Once you go back into Google Calendar app - click your the letter in the top right hand corner. Click on "Manage accounts on this device." You can now see that "System Account" is available and all calendars that you have synced to your iCloud are available in the Google Calendar app.
This will keep everything synced between all the calendars. I use this because I created iCloud shared calendars with my wife, but she is a google user. It does take 10-15 minutes after you enter something for it to sync up on Google, but they will show up eventually.
This drove me nuts for so long I figured I'd post it somewhere where someone was asking to get this out in the world since I could not find an answer to save my life. :) Good luck!