The source code is available at www.gnupg.org

Below is the copyright notice from the README file for the Windows version.

Copying:
========

GnuPG is

  Copyright 1998, 1999, 2000, 2001, 2002, 2003, 2004,
            2005, 2006, 2007, 2008, 2009,
            2010, 2012  Free Software Foundation, Inc.

  GnuPG is free software; you can redistribute it and/or modify it
  under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 3 of the License, or
  (at your option) any later version.

  GnuPG is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
  License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, see <http://www.gnu.org/licenses/>.
