Thankfully, finally some faith in the patent system. This was an asshole move to harass others from using the word GPT in their trade. Kind of like how Instagram did it with the word "gram".
But the US has the same bureaucracy for both. And bureaucracies are leaky abstractions.
It's a huge pain (and pretty much requires lawyers, but you can do it yourself, if you are a masochist), and, in retrospect, not worth it. I suspect that why most companies use ™.
For other people who haven't, see https://en.wikipedia.org/wiki/Industrial_property