Will the transmission for a 2005 Pontiac GTP

Have the original rebuilt if you like the car that much , Replacing an old worn tranny with an old worn tranny makes little sense . There's no cheap way out

Any difference will cause electronic problems. Computer controls are likely different. If both transmissions have the same part number, it should work. Ask a transmission shop for advice; a few bucks would be worth it, but for a ten year old car, its a crapshoot.