Opel GT2 Brochure.The Opel GT2 was a 1975 design concept based on the Opel Manta/Ascona B which came from the Opel Design Centre of which Henery G Haga was the head. The two seater fixed head coupe had a 1.9 litre 4 cylinder OHC engine and boasted a drag efficiency of 0.326.