I've pursued this definitional distinction at a previous company, and got nothing by eye rolls. In modern software design, UI == GUI and design == visual design. That's just how people use those words.

Yes but visual design always includes interaction and behavior. A position that only lays out visual elements with no regard to behavior would be called an artist.

Kind of a pedantic comment given the articles content...

Do designers who design webpages/apps generally call themselves GUI designers?

Pedantic? You described the distinction between UI and UX, you just described it incorrectly.

To answer you question, no. Such people usually design interactions. Behavior. What happens when the user clicks or scrolls or drags. So they're designing UI and they're called UI designers. UX is a step removed, defining the overarching design language for the app and how it makes users feel.

In my experience there are no people who work only on the visual layout of a page and not on behavior (what you incorrectly called UI). Mockup artists maybe, but even they are usually thinking about behavior.