> Isn't "Generative Pre-Trained Transformer" an obscure acronym that openai came up with though?

Nope, they didn't coin the acronym. First appearance of OpenAI GPT was in the BERT paper by Devlin et al. 2018.

But that class of models has been known as generative for years even before the attention paper.

I think that's a weak reason to deny a trademark, given WIMP interfaces were well known before MS Windows.

Yep, and had they applied for a trademark back then, it would very likely have been granted.

Like Apple didn't wait the iPhone 4 to trademark the iPhone brand…

But now the term has been used colloquially for years, so it's out of reach of trademark protect at this point.