The GT6 was a 6-cylinder sports coupé built by Standard-Triumph and based on their popular Triumph Spitfire

Triumph Spitfire

The Triumph Spitfire is a small English two-seat sports car, introduced at the London Motor Show in 1962. The vehicle was based on a design produced for Standard-Triumph in 1957 by Italian designer Giovanni Michelotti...

 convertible. Production ran from 1966 to 1973.
In early 1963 Giovanni Michelotti

Giovanni Michelotti

Giovanni Michelotti was one of the most prolific designers of sports cars in the 20th century. His notable contributions were for Triumph and BMW marques...

 was commissioned by Standard-Triumph to design a GT

Grand tourer

A grand tourer is a high-performance luxury automobile designed for long-distance driving. The most common format is a two-door coupé with either a two-seat or a 2+2 arrangement....

 version of their recently introduced Spitfire 4 (also designed by Michelotti). An unmodified Spitfire 4 was transported to Michelotti's design studios in Italy and late in 1963 the prototype Spitfire GT4 was returned to England for evaluation.