The first version of the GNU General Public License, released in February 1989, was the first operational implementation of copyleft as a legal mechanism. It transformed the intuition behind gnu-manifesto — that software freedom must be preserved through the conditions of distribution — into enforceable license terms. The event release-of-gpl-v1 marks one of the key moments of the founding-gnu-and-fsf-1983-1991 era.
The core mechanism of GPL v1 is what stallman called "copyleft": using copyright law's grant of exclusive rights to the author in order to guarantee, rather than restrict, the freedoms of users. Anyone who distributes GPL-licensed software must make the source code available and must license any derivative works under the same terms. This "share-alike" condition is the gpl-copyleft-mechanism in operation.
GPL v1 addressed a specific problem stallman had encountered: distributors were modifying GNU software into proprietary products by combining it with other code in ways that technically complied with earlier less-formal conditions. The license's "distribution" requirements closed these gaps by requiring that the complete corresponding source code be provided whenever binaries were distributed.
The preamble of GPL v1 is a statement of philosophy as much as a legal recital. It explicitly names the goal: to guarantee "the freedom to share and change free software — to make sure the software is free for all its users." This preamble was drafted with help from eben-moglen, who would become the free-software-foundation's general counsel and the primary legal architect of subsequent GPL versions.
GPL v1 covered less territory than its successors. It lacked the patent clauses later added in gpl-v2, and it did not address hardware restrictions (tivoization) that would be covered in gpl-v3. But it established the fundamental structure — permissions, conditions, and no-warranty disclaimers — that all subsequent GPL versions and many other copyleft licenses inherit.
The practical impact of GPL v1 was initially limited to the gnu-project's own software. The license's wider significance became apparent only when linus-torvalds chose to release the Linux kernel under GPL v2 in 1992, creating the combination of GNU tools and Linux kernel that stallman insists should be called GNU/Linux.