It all depends how serious you want to get, but the first improvement you will want to look at for track work is brakes.
Two pads that seem get good reviews from the Holden guys for street/light track work are EBC Green Stuff and Ferodo DS2500 pads. Some guys flush their fluid and replace with high performance stuff (DOT 5 racing fluid, etc). This is about the minimum most guys do for track.
Obviously as far as rotors you can go aftermarket standard size but slotted, etc (eg. DBA).
Most guys who are serious with track work go to larger HSV spec or aftermarket rotors/calipers (AP, Harrop, Brembo, etc) but it starts getting expensive. Braided lines, etc also. There are already heaps of options available designed for Holdens that will bolt straight on to the GTO.... and the choice/price will improve even more when US companies develop their own kits.
Another possible weak point could be the power steering fluid overheating/overflowing under heavy track work. This is a known problem that doesnt usually occur during street use. Solution... one or all of the following: High quality hoses, forklift fluid (not sure what visc, etc but a lot were using some Mobil stuff that has a higher boiling than the stock fill) and a proper cooler. At the very least check your fluid level and check the tightness of all the hose clamps on the power steering system.
If your driving leans towards the more spirited side and you can handle a slightly rougher ride then a set of lower/stiffer aftermarket springs will improve the grip/handling without costing too much..... obviously if you have the $$$ some decent brand shocks will help as well. Again there are heaps of options with plenty of large aftermarket companies already making parts that will go straight in/on the GTO.
Other than that just make sure all the fluids are topped up and put your foot down
If you have more specific enquiries regarding road course/closed circuit events try posting on
www.ls1.com.au as there are few guys on there who run around the twisties who will glady offer advice/experiences. You may even find some cool vids of Holdens carving up on the track

Alternatively just get on there and do a search for whatever you are after.
Here are some sites to check out for brake, general aftermarket parts and tuning for Holden/GTO:
CAPA
Race Brakes Australia
EBC EBC (Aust Site)
Ferodo
DBA
Harrop Engineering
LS1 Edit Australia