The 1968 Opel GT started life as a concept car back in 1965, that curved steel body was built under contract by French company Brissonneau & Lotz with most of the running gear and other components coming from the humble 4-door Opel Kadett B.
It has been postulated by some (very controversially) that the styling of the Opel GT was a direct influence on the infamous 1968 Corvette which was released roughly a year after the first GT.
The styling is rather similar although you do run the risk of being shot repeatedly if you ever suggest the possibility to an American friend. Just a heads up.
Read more about the Opel GT here.