Initials for the GNU General Public License:

• The GNU Project: http://www.gnu.org/
• some FAQs relating to GPL: http://www.gnu.org/licenses/gpl-faq.html
• compatibility overview GPL v3: http://www.gnu.org/licenses/quick-guide-gplv3.html
• GPL v1 (1989) license text: http://www.gnu.org/licenses/gpl-1.0.html
• GPL v2 (1991) license text: http://www.gnu.org/licenses/gpl-2.0.html
• GPL v3 (2007) license text: http://www.gnu.org/licenses/gpl-3.0.html

See also LGPL.

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GPL Scripts for a discussion on whether it matters if a script.tcl is under GPL.

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