The GNU ("GNU's Not UNIX") General Public License (GPL) was developed in the 1980s by Richard Stallman, an advocate of free software who eventually founded the Free Software Foundation (FSF).
Stallman used the concept of "copyleft" to protect the freedom of GNU project software, effectively reversing the use of copyright law to ensure that source code was never closed off.
This early effort to secure freedom in distributing and viewing the source code has led to the popular Open Source Software initiative.