The GNU 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.
Copyright is a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well.
This early effort to secure freedom in distributing and viewing the source code has led to the popular OSS initiative.