Rebranding a GPL'd app as SaaS

Question

Just a quick question since I'm a little iffy on exactly how the GPL works. Say I am developing a hosted software-as-a-service application, and I've found a free GPL app that does 90% of what I was going to write myself. Can I:

A) Take the code from the app, rebrand it by changing the name and/or logo and, without modifying a single line of code, sell it to people as a hosted service? Would I have to say something like "We are using Project X" with a link to its site? Or does nobody have to know that I'm using an open-source application unless I want them to?

B) Change the structure of the application, add in my own stuff (an extra module that the original app doesn't have, for instance) and not merge the code back into the main branch if the app will only ever be hosted, and not distributed to people?

C) Scrap the front-end entirely and write my own using another technology (Flex, for instance) but use the existing code (possibly modified as with scenario B above) as the back-end?

Can I do any of these? All of them? I'm really not 100% sure but it seems a shame to have to reinvent the wheel if there's an open source app that already does most of what my project would do; it seems a lot easier to be able to take that and add onto it to provide a better solution.

Answer 1

IANAL, nor do I play one on the Internet. Get competent legal advice in your jurisdiction before going ahead with something like this.

A) Yes, No, Yes. B) Yes. C) Yes.

Now, as to whether it's moral to do all those things, well, that's a different question. I would expect that people would probably get a mite upset if you didn't contribute your changes back (it being not in the spirit of the GPL) but providing the output of a program (HTML, etc) isn't counted as redistribution. The AGPL, on the other hand, does have this restriction, to stop exactly this sort of thing.

Answer 2

The GPL requires you to deliver the source code of the entire application under GPL if you redistribute your software publicily. You are not doing this so you won't have to. Companies like Google and Yahoo make handy use of this.

Answer 3

You may find useful the following:

http://stackoverflow.com/questions/493959/creating-software-derivative-works-from-open-source