Generally, if an exhaust is meant for the GTS, it means it will have an extra resonator, and will be tuned a bit differently. it should fit the GT just fine, but it may be tuned more for the 2zz engine, therefore, the gains may not be as good on the gt as, say, an exhaust specifically made and tuned for the GT.
The same holds true for the GTS guys out there who use a GT exhaust, it may give some good gains, or less than adequate ones; but it will be louder than an exhaust made specifically for the GTS for sure.
In either case, the exhaust will fit, but you may be better off with an exhaust made specifically for your application in the long run.
Some may say they will fit either GT or GTS, these may or may not be good, while some are tuned for decent sound and performance on both the GT and GTS cars, others may just make the exhaust around one or the other, and then market it for all models, which would give questionable performance on the type that was neglected during product development stage. So on those exhausts listed for "All models" Be sure to ask questions. Write the manufacturer if you have to; any good manufacturer will usually be happy to respond to tech questions. Ask, "What was the prototype vehicle, a GT or GTS; Was furter testing done on other models? Do you have any dynos?" They will usually reply with some honest answers.
Just my two cents.