Applying a GEGL function via commandline

[Ofnuts]

AFAIK the GEGL tools aren't available in the 2.8 Gimp API. But:

1. There is a non-GEGL brightess/contrast tool in the current Gimp that works just as well (though actually settings could be slightly different). This one is part of the API.
   
2. GEGL filters are also available through a gegl command for command-line environments and scripts (no personal experience)
   
3. The Imagemagick toolbox has a convert command that will take -contrast and -modulate parameters to chnage contrast and brightness.
   

IMHO using ImageMagick is by far the simplest way.