It is most probably a genuine US GTS, given the comments by Sharkey above. But a view of the chassis number on the pedal box is definitely required to make sure it is not another one very similiar to the one Sharkey had. Given that, there appear to be a number of things that are not original, so if that is what you are after you should do your due diligence and comb the Provamo registry to see exactly what it should look like, and read the description of what a US GTS Pantera is on that site. A quick glance at the photos you link to suggests the red paint is not the right color. Given that, I guess it has been resprayed, so the black parts are also probably wrong - as Sharkey mentioned. There are a number of different 'blacks' on a correctly painted GTS. The door mirrors are wrong. The De Tomaso plate on the rear cross member below the transaxle is not original. It has euro rear tail lights, blinker colour should be red for a US car. You need to check the steering wheel has a Ferrero signature cast/engraved into it and the leather is correct - glued on with join on the back side, a Momo prototipo looks almost exaclty the same, but without the Ferrero, and with stitched leather. Airfilter top should be chrome not blue. Gas dampers to hold the decklid up are not original. Looks like brake master cylinder has been changed since the reservoir is not original. As mentioned by the others it seems to have many of the other GTS specific details and a Ghia logo on the front bumper, although it does not seem to have the red background on the backing plate of the logo which it should have.
Cheers, Tim.