If you have multiple calendars in your Google calendar, you can choose which to sync to your iPhone/iPad.
* Open Safari from iPhone.
* Go to http://m.google.com/sync
* Click Sign in with your Google Account
* Enter
Email: me@gmail.com
Password: secret
* Click Sign in
* Click iPhone
* Select/deselct calendars to sync to your iPhone.
* Click Save at the bottom of the page.