Definitely depends on the platform. On Linux it's probably great, but on macOS at least, I've yet to see a GTK app that doesn't feel super clunky. GTK breaks lots of expectations about how things should behave on macOS (shortcuts, standard menus, buttons, etc). Gimp or Inkscape are good examples. On the other hand those are examples of what a "native" app is expected to look like:
https://www.sketch.com
https://paw.cloud
https://www.pixelmator.com
Note that the apps above are not implemented with Qt or Electron either, they use the official Apple frameworks.