The gmail format has changed on my Nexus 7 and I don't know why. What happened?

This may be well known but it was a complete surprise to me. Using Google's device manager, you can actually locate your Nexus to within 20 - 30 metres.
Ring it in the event of misplacing it. Erase ALL data and lock it, and should you have forgotten your password! Don't sweat you can change it to gain operation once again. I myself think Googles security is fantastic and is needed these days.

OK to start with go into settings on your Nexus, tap on Device Administrators. You should now see Android Device Administrators and make sure the box is ticked. Leave this box ticked always for possible future mishaps.

I imagine you already have a Google Account, if not you will have to sign up and do all that is required to obtain one. Using your laptop or PC do the following.
Sign into your Google account and proceed to your account settings. Now look for Recent Activity > Devices
and click on Devices. This then opens up a page showing you your Devices and all activity that you can check on.
Locate your Nexus in the list and click on it, you will see Find your Android devices with Android Device Manager and click on that.
For security Google may ask you to sign in again, after doing that the world opens and you will see a box with your Nexus. The top two icons are for renaming your device or locating, below that Ring, Lock and Erase. Play wisely lol.

This is for those of you that didn't know about this, like myself only an hour ago.