What is GPL?

The GPL was developed by Richard Stallman, the founder of the Free Software Foundation (FSF). It's a popular license for open source software such as WordPress: the FSF's licensing page (checked August 2026) says the GNU GPL is used by more than half of all free software packages.

The GPL grants users the freedom to run, study, share, and modify software. Here, “free” doesn’t refer to monetary cost but to those freedoms: think free speech, not free beer. Anyone who redistributes the program, though, must pass the same freedoms along.

How copyleft works

That redistribution rule is called copyleft, and it's what sets the GPL apart from other free licenses. Anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. A redistributed version must stay under the GPL, and recipients must be able to get its source code.

The rule exists because free code without it can be closed. A program in the public domain can be modified and sold as a proprietary product, and the people who receive that version lose the freedoms the original author gave. Copyleft uses copyright law to make the code and its freedoms legally inseparable.

Know what triggers those obligations before you build on GPL code. They apply when you distribute the covered program or a derivative or combined work based on it; an independent, separate program that merely ships alongside GPL code can stay under its own license. When they do apply, recipients must receive the corresponding source code or a GPL-compliant way to obtain it, and the exact method depends on the GPL version and how you distribute.

GPL versions and variants

The current version is GPLv3, released on June 29, 2007. Adoption isn't automatic, though: projects choose when to move, and WordPress still ships under GPLv2 (or later). The FSF also maintains two related copyleft licenses:

• LGPL (GNU Lesser General Public License): a compromise form of copyleft applied to some software libraries.
• AGPL (GNU Affero General Public License): adds a network term, so users who interact with the software on a server can receive its source code.

The GPL and WordPress

WordPress is released under GPLv2 (or later), and derivative works such as plugins and themes inherit the GPL. WordPress.org acknowledges some legal gray area around what counts as a derivative work, but its position is firm: plugins and themes are derivatives. That shared license is what lets any developer contribute new features, bug fixes, and improvements back to the core software.

For buyers, that position means the plugin or theme code derived from WordPress is GPL, so that code may legally be modified and redistributed. It's why vendors charge for updates and support rather than for the code itself. The whole package isn't automatically GPL, though: bundled assets and independent components, such as images, fonts, or third-party libraries, can carry their own licenses. Check every license that ships with a product before you fork or resell it.

GPL vs. permissive licenses like MIT

Both license families let anyone use, modify, and redistribute code. The difference is what a redistributor must do next:

• GPL (copyleft): derivative works must stay under the GPL, with source code available. Derivatives can never become proprietary.
• MIT, BSD, and Apache (permissive): derivative works may be relicensed, including as part of proprietary, closed-source products.

Choose the GPL when you want every version of your work to stay free. Choose a permissive license when you want the widest possible reuse, even inside closed products. And read a license before you adopt the code it covers; its obligations bind you the moment you redistribute.