I believe that is backwards. The Opel GT was styled chiefly by GM’s Clare MacKichan, who’d played a big part in designing the 1955 Chevrolet before his tour of duty in the Russelsheim studios.
To shake Opel up a bit, GM sent American Clare MacKichan to the German subsidiary in 1962. MacKichan, a pupil of Harley Earl who had led Chevrolet's design studio since 1952, helped refine the design of the 1953 Corvette and oversaw the design of the 1955-'57 Chevrolets and the 1957 Corvette SS.