There are two models referred to as GTS, you have to be careful to specify which model you are referring to.
There was the European spec GTS manufactured from 1971 to ~1984. This car was designed to incorporate improvements beyond Ford's specification, for DeTomaso to offer customers in international markets. This model became the base model after August 1974. When you mention GTS, this is the version that will come to many people's minds.
Then there was a North American GTS. North American enthusiasts hounded Ford to import the GTS (i.e. the European GTS). Instead of giving the North American enthusiasts what they wanted, Ford specified a model for North America they would call GTS that incorporated no improvements, it was merely a Pantera L with a paint and badge job. They were manufactured only in 1974 (i.e. all have chassis numbers greater than 5900) but those with California destinations were redocumented as 1973 models. Bill Van Ess claimed there were 138 of these Panteras manufactured. These are the models Mike described, with THGT in the VIN number.
There are many Euro spec GTS Panteras in North America, so we North American enthusiasts normally differentiate between the two models by referring to one as the Euro GTS, the other as the North American or Ford GTS. Tom Edds and Tony Blevins are two members who own Euro GTS Panteras, Kelly Coffield (Panterror) owns a Ford GTS.