Being allowed to do more things isn't necessarily freedom. For example, is a society where I can enslave people a freer society? I don't think so.
The GPL protects the freedom of software /users/. I the same way that prohibiting slavery protects potential slaves, at the expense of limiting the actions of slavers.
For example, when OS X used GCC all OS X users could get the full and exact source of the compiler their system used. Now that OS X uses LLVM that isn't the case. You can get something close but Apple includes proprietary changes.
It's tiresome to remind people to please remember, GPL tries to ensure user freedoms that might not align with what developers would want, this does not make it less free, it makes it less free for developers.
How is a license that allows someone or some corporation to take some code, add to it, and make those additions proprietary "more open" than a license that specifically disallows that type of scenario? The GPL ensures that derived code stays open while MIT does not. In my eyes, that's "actually open source". I can see the argument that licenses like MIT allow consumers of some piece of code more freedom to do as they please, but that hardly makes it more open.
This gets argued all the time. What the corporation did was freedom for them not the code. GPL protects the code, MIT/BSD protects the people. Depending on your definition of "freedom" you will prefer one over the other.
Here's the obligatory retort that open source != free software: GPL (et al) is about guaranteeing end users' freedom to tinker, as opposed to MIT (et al), which are about allowing developers to use someone else's work in their product without restrictions. GPL enforces a share-and-share-alike ethos, while MIT asserts do-what-thou-wilt. They're not the same end goals.
Why is the implementation language important to you as a user?
To make this a multi-use comment, I'll also reply to GP and say that in every single case where I've benchmarked time sensitive code, GCC has generated faster, tighter code. I still primarily use clang for other reasons, but saying that GCC is more performant seems to be entirely valid.
The reaosn why I vastly prefer clang is for three main reasons.
Firstly, clang and llvm are written in c++ so it's much easier for me to work with. Secondly, it has proper documentation on how to, for example, add new optimization passes or adding a new instruction. This includes tons of talks and presentations on doing this live. Thirdly, the tooling for it is great, such as clangd, llc, lld, clanggtidy, etc.
Gcc seems to have none of this. Oh, and almost forgot, most importantly llvm + clang is actually open source (more open) via a MIT license instead of gpl.