Software licensed under the GNU General Public License (GNU-GPL) entitles anyone who purchases the GNU-GPL licenses to copy and distribute it verbatim. The GNU-GPL is a copyleft license for free software and other works.
Free software and open source software are similar, but not identical. They have different usage criteria. Both can be used by anyone interested with a GNU General Public License. The GNU GPL license ensures that you can request source codes and use them for other software and your own applications. The protection of copyrights aims at the assignment of rights to third parties and the claiming of rights by third parties. Therefore, there are some responsibilities associated with GNU-GPL regarding distribution of copies and modifications.
Developers using the GNU-GPL license protect their rights with respect to copying rights and grant licenses to copy, distribute, and modify the software. For example, the Linux kernel is licensed under the GNU General Public License (GNU-GPL).