The GNX oxygen sensor was originally specified for the GNX because it could withstand higher exhaust gas temperatures. A regular GN sensor shattered during testing and they didn't want to take chances with that ceramic turbine.
May also want to check gmpartsdirect.com for your O2 sensor needs....I haven't checked the price of O2 sensors yet, but I've been able to find some fairly good prices for other TR parts on there.