Will'* seen one, and I've seen 3 or 4 (mostly on E-bay), but no a leather bench seat from a Bonneville won't fit in a 1997 GP unless it is one of the VERY FEW SE'* with bench seats. As Will stated ^ ^ the base mounts are no doubt different, but you might possibly be able to make some adapter plates to use the GP mounts with the Bonneville seats.
I'll go take some measurements on the mounts from bolt-to-bolt on my Bonneville and when my sister get'* home from the grocery store I'll get measurement'* from her GP.
Hopefully I'll have the info available in the next 10 minutes.

Wow Thank you very much!

Well, not so good news.
The Bonneville has 6 bolts for the base-floor mounts. Two in the front and two in the rear, via plates. The GP has 4 mounting bolts, 2 front, 2 rear, via mounting in the tracks, which are tight to the floor in the GP and lofted in the Bonneville. Different tracking styles also, it looks like.
The front to rear (Bonneville outer rear bolts) are the same, but the width on the rear'* is way different. The Bonneville has a 14" width from outer rear bolt to outer rear bolt, and the GP has a 17" width from rear-rear bolt. Forgot to measure the front width, but probably close to the same.
The Bonneville'* front bolts are approx. 15" across from each other.
Good luck with mounts if you do this -if you made a batch of adapter plates you could more than get your money back as there are a bunch of guys from the Minnesota Club GP alone sat in my old SSEi with the lumbar seats and said they'd pay a good penny to have those seats in their GP'*.

Well in this case bad news is better than no news... I was hoping the W-Bodies parts were as interchangeable as the B-Bodies.... I hadn't made any adaptors yet, 1 because I dont know any one with an SSEi :( and 2 I wanted to get a heads up first. (thank you for that) I'm still gonna give this a shot some how, be it Bonny seats or maybe even Buick...

Like your friends say, the GP seats are great, but the could be ALOT better. When I find out how to do this I will let yah know. Thank you very much for the help!