Using the GPL for Eclipse Plug-Ins
The first thing to understand is that the GNU GPL and the Eclipse Public License (EPL) are incompatible. This means that if you create a work that contains code released under both licenses, it's not possible to satisfy the conditions of both those licenses simultaneously. The reason the EPL specifically is GPL-incompatible is because it has its own copyleft terms—they're not as strong as the GPL's, but strong enough that there's a conflict between the two. If you try to release a piece of software with code under both these licenses, you will end up violating one license or the other. More information about compatibility is in our GPL FAQ.
Knowing that the licenses are incompatible, the next question to ask is: are Eclipse plug-ins separate works, or are they combined with Eclipse in such a way to make a single work? After consulting with the Eclipse Foundation to get accurate information about how Eclipse interacts with its plug-ins, we believe that Eclipse and its plug-ins form a single work. That's because Eclipse plug-ins must use some fundamental Eclipse libraries—and the licensing rules for doing that are the same as you would deal with when using any other library — we've written more about this in the GPL FAQ too. The question there is phrased to discuss writing plug-ins for a nonfree program, but the same analysis applies when writing plug-ins for a GPL-incompatible program.
Since Eclipse and its plug-ins combine to make a single work, the plug-in's license needs to be compatible with Eclipse's—and as we already discussed, the EPL and the GPL are not compatible. However, developers who want to release GPL-covered plug-ins for Eclipse can address this issue by providing an additional permission with their license that grants users permission to combine their work with Eclipse in this way. More information and example text are available through that link. Note that, in order for this solution to be effective, all the copyright holders of GPL-covered code in the plug-in must agree to provide the permission. This includes the copyright holders of any GPL-covered libraries or other third-party code that the plug-in uses. Without their permission, incompatibility still exists between their code and the EPL-covered Eclipse libraries.
We hope this information helps clear up some of the questions that people have asked about licensing Eclipse plug-ins. Developers who still have questions are encouraged to send those to the FSF Compliance Lab at email@example.com.