Search google maps for GM Service centers in your area. Find the one with the best reviews. Tell them nothing of your other visit, only that your tranny fluid is grey and your concerned about a blown head gasket and would like it checked under warranty. They should be able to do a compression check. If they have to do the head gasket, they will have to change the fluid. If they say its ok, you should be fine.

As for getting tranny fluid, I would just use Valvoline Max Life Dex/MERC ATF, but if you really want the best you can have the get Royal Purple MAX ATF. At $15 a quart its expensive as hell, and the highest quality fluid you can get. Of course the cheapest ATF at the gas station would probably be good enough. Tranny fluid (except fords and that goofy type F fluid) just isn't the complicated. For the love of god, don't let the dealer rape you for a GM logoed bottle of cheap tranny fluid.

There is no need to drain and refill multiple times. When you drain the tranny its all gonna come out. Sure there might be a few drops of old left, but a few drops in a couple of quarts just won't make a perceptible difference.

Your attitude toward all of this is one I see so often and it drives me nuts. Dealers make auto work sound so hard and complicated that people are scared to do the work themselves. So they pay a mechanic to do a simple job. To the mechanic makes the 16 year old who doesn't care do the easy stuff while the do something more challenging, and its gets done completely wrong or not at all. The owner frets that if the dealer messed it up, it must REALLY be hard. So they keep going to the dealer. When they have the time and the money. Things are missed, parts wear, and cars die. So they buy a new one from the dealer with a warranty to save money. Meanwhile those of us who change every fluid in the car in 2 hours one Saturday a year are getting 400k miles out of used cars we pick up for a few grand. Don't perpetuate the cycle. Make the dealer (ok a different dealer) do what they owe you, and handle the easy stuff yourself.