The challenge of new GTK versions is that they keep simplifying things, because it's aimed at software that's much less complex than GIMP. For instance, the tree views that GIMP relies on for so many things like layer docks are deprecated in GTK4. To deal with that, we'll have to do some redesigns of core features.
There's no intermediate layer in-between GIMP and GTK. It wouldn't be impossible to code, but with the current number of volunteer developers, it would mean further delaying features that most users care more about.