AFAIK trademarks are granted for products. Microsoft was granted that trademark for software. If you opened a window shop and tried to trademark windows you'd fail since it is an existing term in that category. Similarly GPT is a term used in the filed of AI and thus would not be allowed to be trademarked in that category. I'm sure a window shop could trademark GPT for their line of windows.

This is called "descriptiveness", a very bad quality for a trademark application to have.

Microsoft once sued a teenager for trademark infringement for having a website called MikeRoweSoft.com. The kid's name was Mike Rowe and he lost the website in a settlement.

That’s because it is expensive for kids to defend lawsuits, not because a court agreed.

Still redirects to microsoft.com