No, Grand Prix GXP factory 18-inch rims will almost certainly not fit a Grand Prix GT. While both are Pontiac Grand Prix models, there are differences in the wheel bolt pattern (the number of lug nuts and their spacing), the center bore (the hole in the center of the wheel that fits over the hub), and potentially the offset (how far the wheel mounts in or out from the hub). Even if the bolt pattern *were* the same, differences in offset could lead to rubbing against suspension components or the fenders.
You would need to verify the exact specifications (bolt pattern, center bore, offset) for both the GXP's 18-inch rims and the GT's wheel wells to be sure. But it's highly unlikely they would be directly compatible without modifications.
