GTK+ is a popular UI toolkit, used by the GNOME project and several apps (Gimp, Firefox, etc.).
GTK+ is actually a stack of APIs:
Versions for all of these pieces need to be coordinated, so we can think of all of them as a unit, with the GTK+ version "leading the way", so to speak.
GTK+ 2.6 (and related APIs) has been a part of the LSB since version 3.1. In 4.0, we are considering uplifting to as recent a version as we can.
The GTK+ uplift will likely also require adding Cairo to the LSB, since Cairo has been a part of the stack since 2.10.
From the perspective of LSB 4.0 development, the highest version we can include is 2.8, since that's the version shipped by SLES 10.
From a quick look at the Navigator, demand for 2.8 interfaces appears to be low. Unfortunately, the version we actually need is 2.10, as that features some improved integration with printing that the Printing workgroup has identified as crucial. So based on these two factors, it appears there's not enough reason to do an uplift that doesn't really fill any needs.