First things first.... you don't have a celica gt you have a celica st
A gt came with a 2.2L 5s-fe
Second things second you will not gain any hp from changing your exhaust you will simply free up lost horsepower in your currently restrictive system.
Third things third they don't make aftermarket exhaust systems for 6th gen celica's except for the very rare and hard to find greddy sp and sp2.
You're best bet would be to take your car to a specialized shop and have them custom build you a full header-back exhaust system.... 2.0 inches in dia.
Now ready for the fun part.....
since you have an st.... you're going to still be getting 0-60 in 11+ seconds and running high 18 second 1/4 miles.
P.S.
If you happen to have a hatchback there is a guy on my forum willing to trade his 6th gen straight up.
www.6gc.net
He has a 6th get GT with a 3rd gen 3s-gte swap.
The car isn't much to look at and has its rough spots like a ripped interior and a body kit that doesn't match the rest of the car in color........ but it was dyno'd at 260 wheel hp and you can't argue with that.
He's looking for $3k or a swap for a nice hatch back 6th gen so.... might look into that.