How can I create an iCal realtime backup?
Maybe merging iCloud, local, and Gmail all together and then having Time Machine? 

Your thoughts (dare I say paranoia) are interesting and the goal, unique. You could also create a script that runs at a set time interval and fetches your calendars and archives them in a method of your choosing.

Maybe the simplest would be to create an Automator script that will fetch your iCloud calendars and then save it to your ideal location and archive it automatically. The file of your calendar can be pulled when you go into iCal.app and then obtain the URL for sharing the calendar. Copy that URL and change "webcal://p20-calendarws.icloud.com/ca/subscribe/..." to "http://p20-calendarws.icloud.com/ca/subscribe/..."

That should fit the bill and keep system and effort to a minimum.