The way I see it, using GTK reminds me of "native" Mac apps (like the ones written in Swift), but on Linux. GTK sure looks a hell of a lot better than Qt on a GNOME desktop: more modern, polished, and clean. So that's why I'd imagine some people choose GTK.
Ironically GTK apps look better on a QT based desktop than vice versa because KDE/QT devs unlike GTK devs actually make an effort on that front.
You can with minimal effort theme both similarly by installing a theme and icon theme that has versions for both gtk qt but your gnome desktop wont provide a built in gui to configure such, again their choice, their limitation not a limitation of QT. A given app QT app isn't going to pull in a particular theme because that would be the tail wagging the dog.
