Even though iCloud is more feature rich than in the past, Google's cloud services are the choice of many iPad owners. There's no penalty for using Google for calendar, contacts, and mail operation as iOS keeps the iPad up-to-date with Google information.
That's the way it had worked for me for months, with my iPad Air 2, iPad mini 3, and iPhone 6 Plus always having my Google info. Then yesterday, I picked up the iPad mini to check the calendar and none of the Google stuff was there.
A quick look at settings showed that my Google account was gone. It was there not long ago, but when I checked it had disappeared from the iPad mini. It was still there on the iPad Air 2 and iPhone 6 Plus, as was all of my Google calendar events.
I spent hours troubleshooting this odd issue, including adding and deleting the Google Account from the iPad mini settings. Nothing brought back my Google information, which included missing contacts in addition to the calendar info.
I was about to factory reset the iPad mini 3 to see if that would make it start working with my Google account when I ran across a post on a support forum. This guy had a similar problem to mine, and had performed the same troubleshooting step as I had. He couldn't resolve the issue and gave up.
The next time he tethered his iPad to the iPhone using the personal hotspot baked into iOS, he discovered his tablet was once again syncing properly with his Google information. I immediately did the same with my iPad mini and sure enough, my Google stuff was instantly back on the tablet.
There's no technical reason this should make a difference but Apple is apparently doing some sort of magical handshake between the two devices when tethered. None of the technical gurus I've discussed this with have come up with a reason for this solution to work.
So if your iPad suddenly loses your Google information, which is probably a bug in iOS, tether it to your iPhone. It just might bring it back as fast as it disappeared.