Just to be clear, I say that this is the "correct" answer in order to differentiate my answer from the ones that I think are not actually correct.
I often think, and many agree, that the problem is with reactive interfaces supporting both mobile and desktop. Or that the problem is with GUI editors producing disgusting HTML. And indeed these are both problems. But QTDesigner, and friends don't face these problems. Your GTK app isn't going to be a reactive interface, and QT, not using html, isn't at risk of the "disgusting html" problem. And yet these designers aren't terribly popular either.
I often think, and many agree, that the problem is with reactive interfaces supporting both mobile and desktop. Or that the problem is with GUI editors producing disgusting HTML. And indeed these are both problems. But QTDesigner, and friends don't face these problems. Your GTK app isn't going to be a reactive interface, and QT, not using html, isn't at risk of the "disgusting html" problem. And yet these designers aren't terribly popular either.