GEGL was first proposed in 1999, but the GIMP's existing code base has remained in place over several revision cycles since then. As recently as summer 2005, GEGL appeared for all practical purposes dead in the water. Then Kolås took a determined interest in resurrecting the project, and over the next few months he, Sven Neumann, and Michael Natterer studied the code base and got it into working shape again.
It alters drastically the way an image is edited, changing it from a 2D array into a huge set of nodes, called a directed acyclic graph. You can read more about GEGL, which is not yet part of the standard releases and may not be for quite some time.