If you remember, GDR2 was supposed to be out in October but it was later scraped it seems and some of its tipped changes like “Mobile Data Toggle” and Cortana for more arrived with GDR1 version 8.10.14219.314. From the changelog, Windows Phone 8.1 GDR2 looks like the transitory update from Windows Phone 8.1 to Windows 10 for Phones. You can have a look at all the changes that this update brings below. The update can hit normal devices in May-June while developer preview devices may get it in coming weeks.
Windows Phone 8.1 update 2 (GDR2) changelog
Hands-on Demo video (Lumia 540):
Watch Windows Phone 8.1 update 2 (GDR2) in hands-on action on a Lumia 540. We have demoed all the changes that GDR2 brings.
From OEM documentation:
Video Over LTE support
Route an SMS message to a partner application
More Phone languages,
The arrow at the bottom of tiles is without circle and All Apps is written before it.
Settings were redesigned (as we had reported). Settings are categorized under broader headings like we have seen on Windows 10 for Phones.
As per the official documentation, the reset protection feature is only available for new Lumia phones running build 8.10.15127.138. But this feature is not available in all the regions and I can’t see it on my Lumia 640 XL or Lumia 540 (India). Here is how to use reset protection and switch on or off.
Bluetooth Keyboard support finally added.