http://www.gnu.org/licenses/gpl-faq.html

GNU FAQ

Does using the GPL for a program make it GNU software?
Anyone can release a program under the GNU GPL but that does not make it a GNU package.
Making the program a GNU software package means explicitly contributing to the GNU Project. This happens when the program’s developers and the GNU Project agree to do it. If you are interested in contributing a program to the GNU Project, please write to .

Does the GPL require that source code of modified versions be posted to the public?
The GPL does not require you to release your modified version. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.
But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program’s users, under the GPL.

Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.

If I know someone has a copy of a GPL-covered program, can I demand he give me a copy?
No. The GPL gives him permission to make and redistribute copies of the program if he chooses to do so. He also has the right not to redistribute the program, if that is what he chooses.

Does the GPL allow me to sell copies of the program for money?
Yes, the GPL allows everyone to do this. The right to sell copies is part of the definition of free software. Except in one special situation, there is no limit on what price you can charge. (The one exception is the required written offer to provide source code that must accompany binary-only release.)

If I distribute GPL’d software for a fee, am I required to also make it available to the public without a charge?
No. However, if someone pays your fee and gets a copy, the GPL gives them the freedom to release it to the public, with or without a fee. For example, someone could pay your fee, and then put her copy on a web site for the general public.

You have a GPL’ed program that I’d like to link with my code to build a proprietary program. Does the fact that I link with your program mean I have to GPL my program?
Yes.

If a program released under the GPL uses plug-ins, what are the requirements for the licenses of a plug-in.
It depends on how the program invokes its plug-ins. If the program uses fork and exec to invoke plug-ins, then the plug-ins are separate programs, so the license for the main program makes no requirements for them.

If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, so plug-ins must be treated as extensions to the main program. This means they must be released under the GPL or a GPL-compatible free software license, and that the terms of the GPL must be followed when those plug-ins are distributed.

If the program dynamically links plug-ins, but the communication between them is limited to invoking the `main’ function of the plug-in with some options and waiting for it to return, that is a borderline case.

몇개만 퍼옴.

요약하면,

1) GPL에 대한 modification은 가능하고 이를 공개하지 않아도 된다.
2) 팔아도 되지만, 소스를 건네줘야한다. 그리고 이를 구입한자는 공짜로 (또는 유료로) 재배포 가능하다.
3) 내가 수정을 했을 경우, 나의 프로그램은 자동으로 GPL이 된다.
4) 내가 링크할 경우 나의 프로그램은 GPL이 된다.
5) 내가 플러그인을 만들 경우 fork/exec가 아니라면 GPL이 된다.

결국 GPL은 제품을 내보낸다면 소스의 무조건 공개(i.e., 수정 가능성)를 보장해야하고, 배포할 경우 배포 받은 사람이 이를 마음대로 수정 및 공개 또는 판매할 가능성을 보장해야한다라는 것이군요…

최근에 GPL 코드를 수정하고 있기 때문에 마음에 걸렸음.