Hello Naveen,
the original GSR 12000 was shipped with IOS image running on the route processor.
Conversion to IOS XR of a GSR 12000 was possible if the route processor was a PRP-2.
However, there are other requirements that involve the type of linecards supported and the amount of memory that must be installed in each linecard to be able to support migration to IOS XR.
see for example
http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.8/migration/user/guide/up38upinst.html
The XR 12000 is a C12000 running IOS XR on route processor PRP-2 or PRP-3.
Cisco has stopped to support IOS for 12000 and only IOS XR is now shipped with PRP-3.
Hope to help
Giuseppe
