GEGL is a new feature found only in GIMP 2.6+. According to Gimp.org:
“Many highly requested features of the GIMP will be easier to do using GEGL. Layer effects, layer groups, and adjustment layers are quite easily represented (and efficiently calculated) using the DAG organization of GEGL. CMYK and high bit depth support will be easier because GEGL does not make the same assumptions about color spaces and data types that the GIMP does.”
It is not enabled by default. To activate the feature, simply navigate to Colors > Use GEGL and check the box next to it. Once it is activated, you can use the presets available at Tools > GEGL Operations. Approximately a dozen different presets can be applied to layers and images.