The best thing to do is bolt ons. Unfortinately, I dont think you can tune your GP. I dont think they made the software for the 3800 in 2008.
Buy a cheap intake on ebay, or make one from parts at Home Depot. Your throttle body and intake manifold opening is 75mm. Work off that. Make sure you make a FWI. Thats the best you could do, and one of the biggest/easiest upgrades to do to the car.
Next, open up that exhaust. Lucky you, there is no ubend. Exhaust should already be 2.5 so thats not bad. Id look into headers. Every person I know that got headers on their 3800 said they noticed a great deal of improvement.
Want to get a little more serious? Get a cam. Or an easier install, rockers. Gain more top end for a decent price. Now you can always throw on a supercharger from the L32, but I dont know whats all involved when doing this to an L26. But the easiest way with more gains, is to just save for a turbo. But again, you cant tune. Soooo... yeah.