Complex sentence: The P.1 variants have become known as the Gull Four (not "Gull IV"), despite the 6-cylinder Javelin engine, and that before the war the Gipsy Major-powered variant was known as the Gull Major.
Split into simpler sentences: The P.1 variants are known as the Gull Four (not "Gull IV"). That was despite the 6-cylinder Javelin engine, and that before the war the Gipsy Major-powered variant was known as the Gull Major.