Using GPT to write papers is completely unacceptable. This doesn't mean using it at all is bad- I use GPT literally every day for things like Python questions, debugging, or translating code to do a certain thing.
However, every single word in my papers is written by me or by coworkers. So, you're totally correct to be upset with them, that's NOT how this works. Every word on the page needs to be your own, not GPT's.
Edit: because passing another entity's words as your own is plagiarism.