1. Person x wrote a program, that he released under GPL, version 2 or later. In the program it says "Copyright by x". I want to make changes to his program, to remove his name from my changed version and to sell my version. I plan to keep a complimentary link to his site, because his work was useful to me. I will release my program under GPL also. I just want to put "Copyright by me" on the modified copy. Am I allowed to do that? I believe so, from what I've read on the GPL, but I want to be sure. 2. Can someone release the same file with GPL license and a note to it? Like: "The license is GPL, but you must also do ..." (for instance, keep a backlink). I thought that releasing under GPL means that only GPL applies. 3. How can I prove that the file I received had a GPL license? Let's say someone took a proprietary script from a site, removed all its license and wrote a GPL license instead, changed the program's name and put the copy on RapidShare. When I download it from RapidShare, I receive it with a GPL notice and I don't know the name or the license of the original script. How can I be sure the script I receive is GPL'd or not, if there are no links inside it?
1. No, you have to keep the attributions and in most cases, you need to add a description of what changes YOU made. 2. Well, you can modify the GPL, but then it's no longer the GPL.... it's your special license. Which means you can say that it's a license BASED ON the GPL, but you can't claim that the code is governed by the GPL. 3. Generally speaking, you really can't. There are some services these days that offer "indemnification services" that will, in essence, promise the origins of the code. But you're right... there's always the chance that part of any code base might be "borrowed" from someone else unlawfully.
That's right - any derivative works of a GPL work also have to be GPL. If you start from scratch, you can use any license you want, of course, including 'GPL + something.'
Actually, I'm not the one to set the new license. I just received, during the time, some software with GPL license in the package and an extra license note on their site. I wanted to know if I can ignore their note from the site if the license from the package is GPL.
If it is GPLed, then your derivitive work has to be GPLed. The original author's copyrights must remain in place, you should add your own below theirs though.
As I understand it, if the package was originally written by someone else under GPL, and they are distributing it (either "as is" or modified) then the package has to be under GPL, and the extra conditions are invalid. But to save yourself hassle, you might be better off seeing if you can track the package back to its source and get it from the original author, under a "clean" GPL license. If, however, it's something they wrote themselves, then they can set the license to be whatever they want, and 'GPL + something' is valid, if confusing. Since the whole point of GPL is to be GPL, with nothing added or taken away.