I have a lot of things but basically in GT all you need is the car that you already have. You can race with the kit as it comes from the factory but I will only suggest that you get the lower suspension mounts in aluminum. There are some companies that already make these mounts, KingHeadz, YBslow and Max Velocity Racing.
Another thing you will need is at least a good set of slick tires. I would suggest you use the Ipanema rubber tires. These are great tires and should provide you with great traction. You can also change the fuel tank for a 150cc one, the stock one is 125cc. For this you can use the Kyosho tank from the Truggy ST-RR or the tank from the Jammin X1 truggy.
I use a couple of support equipment like starter box, glow starter, fuel bottle, etc. Basically the same as in any nitro class. If you want to go a step further then the sky is the limit.
I also have a set-up station (Hudy and Integy have good ones), if you go with the Hudy like I did you must get the one for 1/8 off road since these cars came from 1/8 converted buggies. A fuel gun is good for quick pits but your pit man must be experienced to use it correctly.
I use a small pit bag. This can be found in any hardware store (I use a Husky one I found in Home Depot). This is to carry everything you need to the pits. In mine I carry my tools, starter box, fuel, glow starter and since we here have a rule that you can't take the car off the track to fix it we also take some spare parts and extra tires.
All this is extra equipment that you don't actually "need" but it will help you when you do go to the race. The good thing about GT racing is that if you have the car you basically have all thats needed to race and have a good time. Good luck racing your GT.