The GIMP source is available at ftp.gimp.org:/pub/gimp.

Copyrights for GIMP-related packages:

gimp
---------
Copyright (C) 1995 Spencer Kimball and Peter Mattis

The GIMP may be redistributed under the terms of the GNU GPL,
Version 2 or later, found on Debian systems in the file 
/usr/share/common-licenses/GPL

The only exceptions are in in plug-ins/common/tiff.c and plug-ins/common/gif.c:

/*
** tifftopnm.c - converts a Tagged Image File to a portable anymap
**
** Derived by Jef Poskanzer from tif2ras.c, which is:
**
** Copyright (c) 1990 by Sun Microsystems, Inc.
**
** Author: Patrick J. Naughton
** naughton@wind.sun.com
**
** Permission to use, copy, modify, and distribute this software and its
** documentation for any purpose and without fee is hereby granted,
** provided that the above copyright notice appear in all copies and that
** both that copyright notice and this permission notice appear in
** supporting documentation.
**
** This file is provided AS IS with no warranties of any kind.  The author
** shall have no liability with respect to the infringement of copyrights,
** trade secrets or any patents by this file or any part thereof.  In no
** event will the author be liable for any lost revenue or profits or
** other special, indirect and consequential damages.
*/

/* GIF saving file filter for The GIMP version 1.3/1.4
 *
 *    Copyright
 *    - Adam D. Moss
 *    - Peter Mattis
 *    - Spencer Kimball
 *
 *      Based around original GIF code by David Koblas. */
/* Copyright notice for GIF code from which this plugin was long ago     */
/* derived (David Koblas has granted permission to relicense):           */
/* +-------------------------------------------------------------------+ */
/* | Copyright 1990, 1991, 1993, David Koblas.  (koblas@extra.com)     | */
/* +-------------------------------------------------------------------+ */


libgimp
-----------
The GIMP libraries may be redistributed under the terms of the GNU
LGPL, Version 2 or later, found on Debian systems in the file
/usr/share/common-licenses/LGPL