I thik that Gimp developers have been working on making sure that Gimp was ready to accept Gegl for a while now. I think that that, along with theme-ing improvements, were a significant driving force behind the development of the last couple Gimp versions. They have put a lot of effort in refactoring Gimp to be ready for Gegl when time comes for full integration.
What your seeing with 2.6 and 2.8 is that Gegl is being introduced gradually. It is currently in a transition process.