They are not the same. I have never seen the inside of the GXPs, but the inside of the GTs basically, it comes in and stays in one perforated pipe...first goes through a small chamber where it leaves the perforations of the main pipe and flows through more perforations in the wall separating the first small chamber and the second, bigger chamber. The perforated main pipe dead ends in the second chamber, and the exhaust then enters a J-pipe that curls it back to the tail pipes. Find that article on the GT mufflers cut away and you will see that a picture speaks a thousand words. GXP mufflers are more of a straight though design, and obviously less restriction. That article states that if the perforated pipe is cut in the second bigger chamber of the GT muffler, it will help create more sound.

I just installed a set of GXP mufflers and I like them...but they are anything but quiet. They are quiet to a lot of people...but not me. They sound sooooooo good though. I am torn...because they are just a touch too much for my desire unfortunately...so I am going to experiment with my GT mufflers and try modifying the guts. I am running stock exhaust except an xpipe where the mid muffler was and I removed the resonators. The GT mufflers are nice...but just too quiet...so I tried the GXPs...and they are a little too much. I can't make up my freaking mind.

 

Let me know if you want to sell those GXP muffs.

 

The first difference you will see is the GXP muffler has 2 outlet pipes. GT's have a single outlet with a dual tip.