So... how was Jetman able to get away with lifting so many elements from Gatchaman? Like... it's very evident.
Homage. It was created as a tribute to the earlier, animated series. Plus, there are enough differences that it didn’t violate copyright. It also helped that one was animated and the other was a live action series.
It wore its influences on its sleeve and no one was pretending it wasn’t riffing on the basic idea of Gatchaman. Still, Kagaku Ninjatai Gatchaman is pretty well known. It was a huge series in the 1970s and had a lot of influence on later works. It’s a cultural institution like Star Trek in the US. To extend the Star Trek analogy, Jetman getting in trouble for being an homage to Gatchaman would be like Galaxy Quest or The Orville getting in trouble for being an homage to Star Trek. You can’t argue that either of those would exist without Star Trek just like Jetman wouldn’t exist without the anime that came first.
It’s not a ripoff or a copyright violation, it’s people who loved something taking elements from that thing to adapt them into what they are working on.
Also, Jetman is a million times better than the eventual live action Gatchaman movie produced years later.
Nothing quite demonstrates the generation divide like how both me and my dad are/were MMORPG players, both of us play lizardfolk monks, but in his case in Everquest the Iksar were legit humanoid lizards whereas mine is an Au Ra kawaii uguu anime girl with a few patches of scales and horns