A document I obtained from GM indicates the only difference between the '04 and '05 GXP is the Gen 6 OnStar. Because the Gen 6 OnStar is dual mode and previous OnStar was analog the antenna is different.
With this change, GM moved from three discreet antennas:
#1. OnStar Voice - Glass mount on the rear window
#2. OnStar GPS - Mounted under the rear deck
#3. XM (DRR) - Roof mounted
and combined these into a single 3-in-1 roof mount unit commonly referred to as a "Shark Fin' style antenna. When I had my OnStar upgraded from the analog to dual mode, I replaced the new glass mount antenna which came with the upgrade kit and changed styles to the '05 model year antenna. This was expensive but relatively easy. In addition to the new antenna, I also needed two new OnStar coax cables. Once done, the '04 is identical to the '05.
StingRay Bike w/cards in the spokes, Weeeeeee!