Just to expound on this point:
> GIMP was very tied to GTK+ 2
People seem to forget that GTK was originally created for GIMP, Gnome came around and co-opted it since it was more free (libre) than QT.
GTK3 was a full rewrite divorced from the GIMP development cycle so broke a huge chunk of things GIMP put in place specifically for their code styling. Thus the long development cycle.
When Qt licensing was a mess there was also the GNU Harmony project to try and create a replacement for Qt.
The problem, like most things with GNU itself, is that large dreams are thought of but never actualized. So, just like using the Linux Kernel vs Hurd, the GNOME project had to plugin what was available; and that was GTK.
Hopefully the now-impending port to GTK 4 will be a lot smoother and won't be such a disruption to their ability to ship the features they've been working on.