(At Remotion) We decided against Qt partially based on my experience working on the Dropbox desktop client—Qt sounds great in principle but in practice we think you're better choosing between native or web tech.
I'm less familiar with GTK.
Qt is a particularly poor experience on macOS. Also the past few years all the new stuff in Qt was QtQuick, which seems basically like JS, and if you're into JS, why not just use Electron?
GTK is a non-starter for something professional, at least if the experiences of the GIMP and Inkscape on macOS are representative. (Also, it's C, which does avoid some ABI problems, but it's hardly suited for an object-oriented environment)
