I'd argue GTK is awful inside of Linux because of client-side decorations (removal of the standard title-bar) that you cannot disable, removal of the menu bar, and the mobile/tablet first design (some people do use desktops and I'm not allergic to text).
I'm pretty sure you can. I like them in most cases, though, just wish they'd gone the Windows/MacOS way of slowly intruding/overlapping a bit occasionally instead of reinventing the whole bar.
Because GTK is awful outside of Linux.