From a theoretical point of view, the Gnu public license is well documented and well known in the public. In contrast to a proprietary license the sourcecode can be redistributed by asking anyone, and it's possible to use the code in the own project if the new project contains of the same GPL license. A possible discussion would read the following way:
user1: Can i copy the Linux kernel and improve it by myself?
user2: Why do you ask?
user1: Because this is not allowed with proprietary code.
user2: I'm not an expert for law, but the GPL license is used by the open source community because it's superior to the plan license which prevents code-redistribution.
This is – in short – the discussion style used by open source advocates. They are proud of the GPL license because it gives them and the world the freedom to share and reuse written code. It's surprising to know that the reality looks a bit different. The legal aspects of coping software are not defined in the literature but what is happen in reality. How many people have copied the Linux sourcecode to their own computer? Not that much. Around the world less than 5 million PCs are running LInux. In contrast, billion of PC are running with Windows. So from a legal aspect, Windows is more legal than Linux. But suppose all the Windows user are not informed, that they can install Linux too. So we have to ask, how many forks of the LInux kernel were created in the past? A fork is a concrete application of the GPL license, it means to use existing sourcecode in the own project. The answer is, that not a single Linux fork is available.
Four years ago, Matthew Garrett (which was involved in the Kernel programming team) has talked about forking the Kernel https://www.zdnet.com/article/matthew-garrett-is-not-forking-linux/ but he hasn't do so. In other projects which are called a fork, a mostly not a real fork, but it's only a patch, which means extra software which can be installed as a plugin to the kernel, similar to writing a mod for a computer game. So the funny question is, why was the Linux kernel never forked? Are the software developers are not informed about the GPL license which explains that this is allowed?
Exactly this is the problem. In the theory, the GPL license allows the programmer to fork the kernel. That means to copy the sourcecode onto the own server and invite other people to modify the content. In the reality, nobody is doing so.
In contrast, normal propiatary software for example BSD Unix is forked very often Apple for example is based on the BSD kernel. It seems, that the GPL license results into a situation in which nobody will fork the code, especially because it's allowed in the license. But what the reason behind it?
Suppose somebody takes the Linux kernel as fulltext, puts it into a git repository and make some changes on it. Then he explains that the fork is licensed under a GPL license and everybody is allowed to download and change the code. The funny thing is, that this user would be the first one in the world who is doing so. That means, it's not sure what will happen next. if Linux is licensed under a GPL License why is no Linux fork available? Open Source activists are explaining to the world that Linux is the future. Ok, which Open Source activist group is powerful enough to fork the Linux kernel and show in reality what will happen?
How open is the GPL license?
Let us explain what the problem with a fork is, if it's done with normal copyright protected software. Suppose somebody takes the sourcecode from the Microsoft windows ooperating system, copies it into a github folder and invites other people to send a commit. The other user will clone the repository, create patches and commit it back to the github folder. From a technical side such a project is a great idea. That means, the programmer will improve the code a lot, fix all the bugs and the end user can download the next release.
Everybody who is familiar with the history of the computer industry knows what the real problem is. It's not the technical problem of create commits and push the result to the server but the problem has to do with the license, the copyright law, and that forking of copyright protected content is a form of redistribution. It's not very complicated to predict that forking the Windows sourcecode will produce a lot of trouble in theory and in reality as well. That is the reason why nobody is doing so.
And now let us imagine the same situation for the Linux kernel. Everybody knows that Linux is a different project. So it's allowed to create a git project publicly and commit changes to the server? I don't know. From a theoretical point of view, it's allowed because this is explained in the GPL license. But in reality, nobody has tried to do so in the past, so it's not clear if it's allowed or not. What we can say for sure, that coping the LInux sourcecode to an online repository and invite other people to commit patches is equal to fork a project. Forking is something which is discussed by the copyright law and if it's not allowed it will produce a lot of trouble.