[Lizard]

Hi Omar,
(There might be shorter way, but this works for me...)
You can publish a calendar from OmniFocus. On the Sync tab of Preferences, tick the box for
[ ]Publish Due reminders as a calendar.
Then if you tap the "Subscribe in iCal" button next to it, you'll get the calendar in Calendar app.
Once you've subscribed to the calendar in Calendar app, right-click it and choose "Get Info". This should be the URL you need.

However, if you're specifically planning to subscribe from Google, last I checked, Google's calendar system could not subscribe to a calendar that requires a password. If you're syncing with Omni Sync Server, the calendar will be password-protected (just like your OmniFocus data), so Google probably won't be able to access it.