Using my iPad with IOS 7.1.2, how can I sync my calendar from my iPad, iPhone, iPad mini to Google claendar?
If you add your Google account in the "Settings" app under "Mail, Contacts, Calendars" the sync works in both directions: Events entered on http://calendar.google.com appear in the iOS calendar app and events saved into the Google calendars on your iPhone/iPad will appear on the Google Calendar website.

However this works only for new events. If you have existing events that you would like to copy into Google Calendar you can use my free app SyncCal (iTunes Link).