Well from what I do remember of seeing Navy and Marine Corps birds at airshows from that era was that GullGray did have those tonal variations, especially depending upon the light. There is just the hint of a tan in there I always thought. I prefer Humbrol's Satin or Gunze's Aqueous Semi Gloss shade of Gull Gray myself. But a dab of Deck Tan and a bit of White added to the Tamiya Sky Gray will give a reasonable approximation.