Thanks Maken for the Tip. Nokia has informed to Nokia Developer community that earlier reported GDR2 update with Build 1232.5957.1308.00XX is already being shipped on new Lumia 620, 820 and 920. For currently in use Lumia 620, 820 and 920 it will come as a software update later. Following is the change log according to the article.
For Nokia Lumia 920 and Nokia Lumia 820:
This software, version 1232.5957.1308.00xx, includes further performance and stability improvements, enhancements to display stability during a call, and improved automatic display brightness adjustment.
Apart from this as reported by users there are improvements in sharpness and color balance in images captured with Lumia 920’s camera and option to manage the other storage.
For Nokia Lumia 620:
This software, version 1030.6407.1308.00xx, will include improved touch behavior and enhancements to system stability and camera picture quality, such as corrected camera exposure when the flash is used in bright light conditions.