Googling “gt500" yields a link for the 2017 GT500, the “Cobra of Performance Cars,” though it redirects to the regular Mustang page. Apparently, the page’s code also references a 5.8L V8, i.e. the Trinity motor.

Reminds me of that one time Ford had a reference to an “Explorer SHO” that turned out to be the Explorer Sport. Although, there was also that listed curb weight for an automatic/DCT Fiesta ST that never panned out...