* Ford Mustang GT: This is a muscle car, a relatively affordable (compared to the GT) high-performance version of the Mustang. It's a two-door coupe or convertible, with a V8 engine.
* Ford GT: This is a supercar, a very limited-production, extremely high-performance, and very expensive car. It's designed for speed and track performance and uses a powerful twin-turbocharged V6 engine.
Think of it this way: a Mustang GT is a powerful, sporty car for everyday driving (though performance-oriented), while a Ford GT is a highly exclusive, track-focused machine. They share the Ford badge but little else in terms of design, engineering, or price point.