US GTSs were made late in the 74 model year only and then only in very small quantities, less than 100. It was the end of the Ford era and they did not sell very well; most ended up selling in 1975. Some were retitled to 73 for California delivery. The GTS VIN is the best way to tell.
They were exaxtly the same as all the rest of the 74Ls except they weighed more. They added the riveted on fiberglas flares, the semigloss black hood and deck lid, and the GTS graphics on the side of the car. That is it! European GTSs, now that's a whole different story but once again, extremely rare in the US.
Based on the number of cars that claim to be GTSs, you'd think they made a thousand of them.