Goju and Aikido are much more the same than different. That does not mean no different - just very very similar. Again, every technqiue in Aikido will be found in Goju Ryu.
The difference between Aikido's execution and Goju's execution is in the Aiki.
This is the same difference between Aikido and Jujutsu.
Contrary to popular belief, ground fighting is not something new. It has been iin Jujutsu for centuries, Judo from the beginning and Karate from the beginning - including Goju Ryu. Just because some teachers either never taught it, or never figured it out for themselves, or chose not to teach it, does not mean it never existed before. If you fully explore that kata and understand what you are doing you will see an awful lot of grappling, throws, sweeps, joint locks, chokes, and a host of things that can be easily applied on the ground.
Not just in Goju Ryu, but in relation to all Okinawan karate, I had always been told that "a kick is a sweep and a throw", "a block is strike is a throw" "blocks" always become joint locks, chokes or a throw of some sort.
And these techniques are just as easily applied on the ground as standing.
I seriously doubt that Miyagi did not teach technqiues on the ground, as I am certain that he was taught. I also think it odd that 3 different teachers from 3 different lines of Goju in 3 different parts of the country just happen to make up the same ground applications from the kata. I'm sure they learned it from some where. And the only common link is Miyagi himself.
Yes, the rules of Karate tournaments changed since the 60's, but we also started seeing new American "kata" being created and the even experiemntation with musical kata. Even in Japan karate changed drastically in the 60's as tournaments became more popular.
Before there were tournaments there were no rules at all to fighting. The addition of any rules was a drastic change. As soon as the focus turned to the entertainment value of the kata performance, that was the beginning of the end for bunkai. It was never completely lost, but it was seriously endangered. By the time Shogo Kuniba started teaching Kata here in the late 60's-70's there were a lot of people that never heard the word bunkai.
His ideas seemed all new to people then, but he was teaching pre-WW2 Karate. Then it seemed to die down some and now again people are "rediscovering" the ancient kata. Perhaps the common notion that ground fighting is new is yet another sign that the older arts just are not fully understood.
One more thing:
Goju Ryu can be seen as primarily a standup art. That would be because the objective is put your attacker on the ground while you stay standing.
Going to the ground yourself when you don't have to is foolish and dangerous. But even karateka of old had to have some defense in the event that they did get knocked down.
At it's most basic levels Goju Ryu is a standup striking art. As students get more advanced they learn the grappling applications. Then the more advanced students learned the ground fighting. Those that I learned the ground applications from decided to start teaching it sooner because of the growing popularity of BJJ. But as I said, it has always been there.