Having owned / DD'd a GXP for almost 10 years, I've got one piece of advice:

If it doesn't come with documentation of an upgraded trans from a known 4t65 builder, i wouldn't touch it. There's a reason so many of those cars sell for dirt cheap - no one wants to be the one stuck with a $3-5k transmission bill (depending if you've got to pay labor / shipping).

When it does happen, many people can't stomach putting 100% of the cars value back in the trans, so lots of people just get a cheapo rebuild done to offload the thing, only to pass the problem onto the next person.

 

That's why I said might. Even if I bit one off, I at least have someone in my corner who knows how to do the work. (my Dad, who rebuilt the 4T65E that came in my Mom's car from the factory to handle an L67 swap. He didn't wanna use the HD transmission because of less aggressive gearing in it. He's also going to be rebuilding the 4T65E-HD in my grandma's 05 Impala SS soon). Either way I still gotta worry about my own car. I don't know if anyone's ever been into the transmission, and the 60E is even more of a piece of **** than the 65E.