I have seen photos of the Nissan design team working on their designs and on the walls around them are the various sports cars of the day. I don't recall if I saw a GTO among them but I would be surprised if it was missed. There were certainly 911s, Triumphs, Jags, Corvettes, Mustangs. So if the GTO was missed I would not understand why. Plus there is the similarity between the 240z wheelbase and the GTO. Coincidence?
I wouldn't go so far as saying the engine and gearbox specs were based on the GTO. A five speed box was the best to be had at the time so it was natural to gravitate to it. As for straight six, Nissan inherited the basis for the L series engine when they merged with Prince. Prince had been developing the inline six for a few years in the skyline. So any similarities to the GTO drive train are probably by chance.