Antennas are specific to technology. The antenna you have on your window is for analog OnStar. The '05 GXP has dual mode technology and in essence (without getting too technical) works primarily in a different frequency range. The '02 - '04 Bonnevilles also use analog, but if the OnStar is upgraded (only the '03 & '04 are eligible), the antenna is also replaced (albeit another similar looking glass mount antenna). Antennas are tuned to specific frequency ranges. It may not be worth your time or money as you won't be able to use OnStar services after December 31st anyway. You may want to consider just removing it if this is a cosmetic issue.
StingRay Bike w/cards in the spokes, Weeeeeee!