The Linux kernel was re-licensed under the GNU General Public License in 1992. Linus Torvalds made the change with Linux version 0.12, released in January of that year.
Before the switch, Linux used a license written by Torvalds that restricted commercial redistribution. Moving to GPL version 2 made it possible for people and organizations to distribute, modify, and sell Linux, provided they followed the license’s source-code and redistribution requirements. The change helped Linux attract contributors beyond its original Minix and university communities.
A common mix-up is confusing the kernel’s 1992 licensing change with Linux’s first public release. Torvalds announced Linux publicly in 1991, and version 0.01 appeared that September. The GPL change came afterward, in version 0.12. Another distinction is that the Linux kernel remains specifically GPLv2-only, rather than automatically adopting later GPL versions such as GPLv3.