Good info from these guys so far.
Biggest limiting factors for the GTO are the stock brakes, front end geometry, and limited tire size.
For the brakes, stock up on the cheapest rotors you can find (I found some for $8 each and bought a dozen of them), a set of track duty pads (Carbotech XP10, EBC blues, etc ...), and get some good fluid into the lines (I use Wilwood EXP 600).
Rotors are consumable items when you track this car, so DBAs are not the way to go unless you're ok with spending that much cash every couple of events.
Keep the DBA rotors and EBC red pads for street duty, and swap them out for track events.
For the front end geometry and tire size, you're pretty much limited to .5 -.75 degrees of negative camber with the stock configuration and 245/45/17 tires (are you on 17 or 18s?). To get more camber, you'll need different rims with a better offset, or some wheel spacers.
I've been tracking my GTO for almost 3 years now, and go to 5 or so events a year. Each event is 2 days, with four 20 minute sessions per day. I've run the EBC yellows, Blues and Oranges, and liked the blues the best. My next event is this weekend, and I'm going with Carbotech XP10 pads this time.
After you have a few events under your belt, and the tires start limiting your times, move up to a comp-R tire (I run Nitto NT01s).
Believe it or not, the stock suspension on these cars is very good on the track. It's predictable and easy to control, especially for someone who is learning. The biggest draw back from the suspension is the understeer. To dial it out, you can swap the rear sway bar only, and leave the stock front bar in place.
My car right now has stock springs, Monroe Sensatrac replacement dampers, SLP rear sway bar, all poly bushings and 6mm spacers up front. I'm able to get neg 2* camber up front now, and that combined with the SLP rear bar, completely eliminated the understeer. The car feels fantastic now, although I'm not sure how well I would have done 3 years ago if the car was setup this way.
The understeer is safe, and will keep you on the track when you enter a corner too fast. You'll turn, and the slip angle will be huge, causing you to scrub lots of speed ... eventually you'll end up taking the corner wide, but remain on trackl. The same situation in an oversteer car, can easily cause you to spin before you realize it.
Another thing to watch for is oil pressure ... the front sump design can be problematic with high lateral G forces. Get a good gauge and look at it each lap in a safe spot (long straight). Also put an extra quart of oil in before an event to help control the oil starvation risk.
Good luck!
Oh, and I forgot to mention: Sliding is slow ... if your drifting, or spinning tires, or sliding, you're losing time. This is not a bad thing if you're wheel-to-wheel and trying to prevent someone from passing, but in time trial (or HPDE), it's just going to eat into your time. Watch a small, light, low HP car go around the track. That's called a "momentum car", and a good driver will squeeze every thou of a second out of it by using minimal braking and setting the car up to be on the gas as soon as possible mid-corner. If you come flying into a corner with all your Horses pumping, and can't slow down enough, it will feel fast as you slide through the corner, but reality is you can't get on the gas until you have control again and are setup to ride the angle out to the exit berm. The guy in the momentum car will come in casually, brake a little bit, and glide through the corner, getting setup to be on the gas just before the apex, and blow by you on corner exit; and because of this, his momentum will most likely prevent you from catching him on the straight even though you could have 200+ HP advantage on him.
The sayings "Smooth is Fast" and "Slow in, fast out" are absolutely true!