I was reading around because I'm planning on doing a pan drop and filter replace (Just hit 50,000) and from what I've read it's supposed to be a maintenance free transmission. But I think what that really means is they want you to go to the dealer to get it serviced, hence the missing dipstick.
A filter replacement is a good idea every 50k IMO and maybe a complete flush every 100K
I'd say to get your fluid flushed, regardless of how the car has been driven. It'll replace more of the fluid. But I'm not sure if they still go in and replace the filter. I would think so but I never had a car long enough to decide to do a complete flush.
Doing a pan drop and filter replacement will replace 6.9qt of fluid.
Doing a flush I believe will replace 9.5qt
And when the transmission is first built with not a single drop inside. It takes 12.9qt to fill it up.
After whatever job is done you then need to have the car on a level surface. They say to raise it up, but I think you can do it while it's on the ground because of where the check plug is located. But as you fill you have to crack open a little check plug and just go slowly until fluid drips from there, that's when it's full and you're done.
I got a majority of my info from the Malibu forums. Seeing as the Malibu is the same car, I figure it's well worth following.
And according to my Chiltons for G6 (Worth the $15) The 2005 G6 used Dex3 fluid and 2006 and up use Dex6.
As an added touch I even added a trans cooler to help extend the life, Heat is one of the most common causes of transmission failure. Not to mention is also made my transmission perform pretty nicely.