Originally I posted this as a question in the maintenance forum. I've since replaced the cooler lines successfully and will describe the procedure here.
Background: Found transmission fluid leaking onto the ground directly underneath of the radiator. Removing the lower engine splash shield that is attached to the bumper revealed that the transmission cooler lines were rusted through and the fluid was pooling on top of the splash shield before leaking to the ground. This G6 had 71K miles on it during the time of the leak.
Depending on how well you know your way around the g6, the job could take anywhere between 2-4. My familiarity with the car prior to this job only included oil changes, brake pad/rotor replacement, tranny filter/pan gasket replacement, and spark plugs so it took me about 4 hours.
Replacement part was ACDelco 15212982
1. Remove the lower engine splash shield under the radiator. It's held on by 4 plastic clips and 5 screws.
2. Remove the nut securing the cooler line bracket to the transmission. Pull the bracket/lines out of the transmission and have a catch-pan ready as some fluid to come out.
3. Detach the white retainer clip holding both lines to the radiator support.
4. To access the nut connecting the lower cooler line to the radiator, remove the radiator support bracket that is directly below it. The support bracket is attached to the frame with two bolts. After removing them you can twist the bracket out of place. This can be a bit of a pain because there's a plastic panel immediately to the right of the bracket that restricts its movement. Removing a few plastic clips should help and allow you to pry the paneling back while you twist the bracket out of place. Once you've gained access to the nut, remove it while having your catch-pan ready.
5. Next you'll need to reach the nut securing the upper line to the radiator. If you remove the outlet duct you can easily reach it from the top, but to remove the nut you'll need either a crowfoot wrench or something similar that will allow you to unscrew it from the awkward angle. Since I only had regular open wrenches available, I removed the right headlight assembly to gain easy access to it.
6. Pull the old lines completely out from under the car. Next I removed the two radiator connector nuts from the old lines to reuse with the new lines. First remove the black plastic retainers by easliy prying them off. They will expose the metal retaining clips that secure the nuts to the lines. These can be removed with tweezers or a pick. Once both retainer clips are off, twist the connector nuts off of the lines.
7. Install the connector nuts onto the new lines and secure them with the existing metal retainer clips. Finally, slide the black plastic retainers over the nuts (new cooler lines should include new plastic retainers).
8. Fit the new lines into position from under the car. I installed the transmission-side bracket first, then attached white clip to the radiator support, followed by the lower radiator nut, and finally the upper nut. Be aware that when you go to screw the lower nut back into the radiator, the fitting that it threads into will give way when you press on it and fluid will drain out until you manage to fully screw the nut back in, so have the catch-pan handy.
9. Re-install any parts you removed (e.g. radiator support bracket, splash shield, etc).
10. Remove the 11mm transmission plug to get ready to check fluid level when topping off ATF (Dexron VI). Remember to top-off while the car is running.