Er, what? I ran Gentoo from 2001 until mid-2013, and I was a maintainer and core developer of parts of Xfce (a Gtk-based desktop environment) from early 2004 until late 2009, and I have no idea what you're talking about.
Gtk was a pain to develop with, sure, but only due to the awkwardness of GObject's attempts to build an OO system on top of C.
From a user perspective, I never saw the problems you're describing. Even having built Xfce out-of-tree all the time, I never recall having to do a full rebuild due to a Gtk update.
I've since moved on; it's a shame to hear what the OP is saying about 3.x, but 2.x most certainly didn't have these problems that you describe.
I'm talking about the change from Gtk1.0 to Gtk2.0 (or that's what I remember mostly). I gave up on Gentoo around 2001 or so. Things were better when I left, but there was still breakage whenever I did a world-update. I had fallen into the lazy habit of not updating often enough which contributed to the problem, but still, the promise was that all you had to do was to do a world-update and everything would work fine. Maybe it does now, but I remember having to run a crippled system for a couple of weeks at a time, while trying to sort through the numerous broken packages. I had used many different desktops including XFCE. They were all pretty good and completely flexible under Gentoo, as the Desktop designers had intended. That part was fine, but the underlying Gtk libraries were a complete PITA whenever there were large changes.
I'm still a bit confused. The 1.x to 2.x transition was intended to be a breaking change (hence the major version bump). I don't see how you had application issues during that, as 1.x and 2.x were parallel-installable, and apps had to make a conscious decision to upgrade.
I don't recall the stability of the 2.0.x series, though: it's possible they didn't get things right and were still making breaking changes even though it was the stable series. I don't remember that being a problem, but I'll admit my memory of that period isn't perfect.
I do remember some app developers prematurely upgrading and releasing versions that depended on early development releases of 2.0 (and later, on the 2.1.x unstable series), which often did cause breakages. But that was really the app developer's fault for depending on versions that made no API/ABI guarantees.
Sorry, I don't recall the particulars since it was a long time ago. But believe me, there were lots of problems with the libraries. I still cringe whenever I have to deal with a Gtk library.
Gtk was a pain to develop with, sure, but only due to the awkwardness of GObject's attempts to build an OO system on top of C.
From a user perspective, I never saw the problems you're describing. Even having built Xfce out-of-tree all the time, I never recall having to do a full rebuild due to a Gtk update.
I've since moved on; it's a shame to hear what the OP is saying about 3.x, but 2.x most certainly didn't have these problems that you describe.