I think the problem isn't defined clearly enough to be discussed. GTK and QT
are a collection of different things. GTK includes a rendering framework
which is pluggable, and indeed there is a QT engine plugin, or at least
there used to be). I used it for a little while as I tend to prefer GNOME
and GTK applications to KDE ones but I prefer the appearance and speed of
QT's rendering. It proved to not be particularly stable / didn't play well
with certain themes etc. which I suppose was inevitable. But, should there
be a decision for GNOME to migrate to QT, it might be a first step.