A flush is never a bad idea, as long as its maintained. As a "GM Engineer" I'd imagine it was serviced once before you got it. So it would be ok to have a flush done. The trannies, especially in our cars, are recommended to be serviced (pan drop, replace filter and fluid) every 30-50k. When they go 100k+ and then get a full flush, they see problems immediately after.

They are both 4T65-E transmissions in the GPs, the only difference is the GXPs get a couple hardened parts to handle the extra power of the V8 motor. Same problems tho if you don't maintain them.