Back in December Nokia started rolling out the Portico (build 10211) software update for the Nokia Lumia 920 for AT&T. This update later continued to roll out for the Nokia Lumia 810, Nokia Lumia 820 and HTC 8X devices through various carriers. This update was internally referred to as GDR1 (General Distribution Release".Now it looks like Microsoft may be readying another update before they eventually release Windows Blue, and it is known as GDR2.
The GDR2 update is expected to arrive this summer, and there are additional reports which suggest a GDR3 OS release will also follow. With Windows Blue expected to arrive later this year, it is important for Microsoft to show their customers, they are committed to continue to improve the Windows Phone platform with updates and improvements.
It also looks like Microsoft will continue updating their operating system under the Windows Phone 8 mantel without rebranding the OS to Windows Phone 8.x until their next major release.
source - WPCentral