Eels are fish

While reading an article, I went to check how an eel differs from a lamprey - and I found out that a lamprey isn’t actually a fish

"Lampreys /ˈlæmpreɪz/ (sometimes inaccurately called lamprey eels) are a group of jawless fish"

I'm not sure what you mean? Jawless fish are pretty far from most fish but that's not much of a reason to say they're not fish.

GPT-5 says that now its not. So while lampreys are technically fish under traditional definitions, modern evolutionary science places them as one of the most primitive branches of vertebrates—not part of the “true” jawed fish group.

I don't know where it got that idea because I only see a few people using the term "true fish" and they're using the normal definition of vertebrates. If you want to be extra restrictive then there's multiple options. If you really want to exclude tetrapods you might use rayed fish. And there's also bony fish if you think sharks aren't quite right having only cartilage. Who uses jawed fish in particular?

Gpt makes things up. It really shouldn't be used in technical discussions except for a catalyst to find something to look up.