Is this only in OmniFocus or in your iOS Calendars app as well? If you're not seeing them on your iPhone at all, it's probably because you haven't enabled the other calendars for syncing. How to do this depends on whether you're using Google Sync (Exchange ActiveSync) or a CalDAV (via a Gmail account in iOS or a separate CalDAV account).
For Google Sync, go to
http://m.google.com/sync
If you're using CalDAV, go to
https://www.google.com/calendar/iphoneselect
More information can be found at the following links:
For Google Sync:
http://www.google.com/support/mobile...?answer=138740
For CalDAV:
http://www.google.com/support/mobile...?answer=151674