The GNU General Public License was created by Richard Stallman in 1991. This license allows users to use, change, copy, sell, and give away the product provided they follow two key terms of the license:

• A copy of the source must be included, or written instructions on how to get a copy of the source must be included.
• The license cannot be changed or removed. Once the GPL has been used on a work, it, any copies or derivates must always use the GPL.