Some headers are not street legal, BTW. Most (save for the DC) are not carb approved, and are considered race-only parts.
Those headers with the theta pipe (that little metal divider plate at the flange) are street legal (Trial, DC and TRD
have the theta pipe)
if it has dual runners going to the flange, it will not be street legal; the Theta pipe is what allows the catalytic convertor to heat up enough to be effective. If the Theta pipe is missing and it's just dumping into the cat pipe, the cat will not heat up enough and your cat will clog or you may very well fail emissions.
This is also why the factory header was designed with that divider plate at the flange.
Hotshot does make good headers, so if you own, or know someone who owns a GT, and are looking for a header, I'd recommend the hotshot. Not for GTS though, like others have said already.
I believe Midnight Performance also designed a header, but it was never marketed. the prototype went up for sale on e-bay not too long ago.
And TPR swears they will make this header, that's the one I will probably get, assuming it gets released eventually and it won't make my car fail emissions (which is entirely doable, since I live in Illinois)