I think you may also be misunderstanding the GPL here, though.
If you obtain a copy of a project under the constraints of a GPL license, then, if you distribute that project, or a secondary project built using that project, you must also provide the source (plus modifications) alongside the project. There are some caveats about what "built using" means, and what it means to distribute a project to users, but this premise is broadly true for most forms and variants of GPL-family licenses.
The thing that allows relicensing to work is that GPL applies to the distribution of software. Linux isn't, per se, "GPL code", but rather when I download a copy of the Linux kernel, I'm downloading that under the provisions of the GPL license. But the person who wrote the source code does not have to licence the project to themselves. And they can choose to distribute further contributions to the project - or indeed the original contributions - under a new license.
Where this gets a bit more complicated is that most open source projects made up of contributions from a variety of people. Each of these contributors owns the copyright - and therefore distribution rights - to their code. For this reason, many organisations require you to sign a CLA to contribute code to a project, which signs over some rights to the project itself (or its core maintainers) and allows them to distribute your code under their choice of license. Alternatively, some projects will ask all contributors to a project for permission to relicense the code, and rewrite code where they couldn't get permission.
So the statement you make, that all future modifications of that code must also be GPL, is untrue. It is definitely the norm, because relicensing code is complicated and rarely done, and people who use GPL seem mostly satisfied with continuing to use it indefinitely. But it is perfectly legal for an organisation who owns the copyright to a given project to relicense that project under a completely different license. (Although they cannot revoke the original licence for the code they've already released.)
If you obtain a copy of a project under the constraints of a GPL license, then, if you distribute that project, or a secondary project built using that project, you must also provide the source (plus modifications) alongside the project. There are some caveats about what "built using" means, and what it means to distribute a project to users, but this premise is broadly true for most forms and variants of GPL-family licenses.
The thing that allows relicensing to work is that GPL applies to the distribution of software. Linux isn't, per se, "GPL code", but rather when I download a copy of the Linux kernel, I'm downloading that under the provisions of the GPL license. But the person who wrote the source code does not have to licence the project to themselves. And they can choose to distribute further contributions to the project - or indeed the original contributions - under a new license.
Where this gets a bit more complicated is that most open source projects made up of contributions from a variety of people. Each of these contributors owns the copyright - and therefore distribution rights - to their code. For this reason, many organisations require you to sign a CLA to contribute code to a project, which signs over some rights to the project itself (or its core maintainers) and allows them to distribute your code under their choice of license. Alternatively, some projects will ask all contributors to a project for permission to relicense the code, and rewrite code where they couldn't get permission.
So the statement you make, that all future modifications of that code must also be GPL, is untrue. It is definitely the norm, because relicensing code is complicated and rarely done, and people who use GPL seem mostly satisfied with continuing to use it indefinitely. But it is perfectly legal for an organisation who owns the copyright to a given project to relicense that project under a completely different license. (Although they cannot revoke the original licence for the code they've already released.)