I have found the app, which uses the code, released under GPL3 license, however, the app itself isn't open-source. For the last two days tried to find a reasonable answer but couldn't find any. Might it be illegal or I am missing some data here? I always thought that if GPL3 code is used within the app then the app should also fall under the same license.
If the app is distributed to other users and contains GPL-covered code, its source must be published under the same (or compatible) license.[source] This is one of the basic principles of GPL.
If however the entire app runs on server or is only used internally in an organization, it doesn't have to be open-source. (This is different in the GNU Affero GPL)
Using an unmodified GPL-covered compiler/editor/tool to compile/write another program without open-sourcing it is legal.[source]
I will preface this by saying your question feels like it belongs in a legal advice section, so please understand that I do not come from a legal background but a software background. I disclaim any personal liability from my advice given. My advice may be entirely incorrect and very likely is at least partially incorrect.
As the prior commenter MadHatter pointed out, this is not a cookie-cutter thing you're seeking, which is likely also why your searches didn't give you the answer you hoped for.
When dealing with open source in general, the rules differ between each license - sometimes by a little, sometimes by a lot so it's good that you at least know to ask regarding GPLv3 specifically. Having said that, it's important that you understand each licensee has the freedom to provide exceptions and clauses, so just because something is labeled as GPLv3, you need to still look at the license agreement for exceptions and mixed licenses - for example, some of the code base being in a different license than the rest.
Regarding contribution, the GPLv3 license essentially requires that you make available any source code changes you have made to that code, and you also release it under GPLv3.
It is my understanding that if the parent application uses code that is licensed under GPLv3 but the app itself is not licensed that way, they must do the following:
- Distribute the GPL license with the GPL licensed code when it is distributed (in other words, if it is distributed with the app, they have to also distribute the license)
- Make available all modifications to the GPLv3 licensed code (if any), and those modifications must be released under the same license.
- Changes must be documented
There is of course much more to the GPL than this, but this is my attempt at summing up the important part you were asking about. So as I understand it if the source code was not modified/adapted to work with their product, and the license is distributed with it, then they have not violated the GPL.
An open source app is one where the source code is publicly available to anyone. While the GPL allows you to freely share the code, no part of the GPL forces you to make the source code publicly available. If you sell a binary program that uses GPL code, then you either have to provide a copy of the code with the program or give the user instructions on how to get a copy of the code. Only a user of the binary program must get access to the source code, providing access to any other person is at the discretion of the developer.
If they are selling a GPL based program and not providing users with access to the source code then they would be in violation of the GPL. It may be that you won't know this unless you purchase a copy of the program.
From GPLv2 -
You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange;
Another consideration is how they use GPL code. If the program uses a GPL library and they only share the library code, then they could get into a grey area depending on how the program is packaged with the GPL based code.