This[1] is the one I used as a reference for my kernel build which includes brcmfmac
There is also a feature called 'channels' (not widely documented AFAIK) which allows you to describe an outside repository of code you can reference to install packages. There is one[2] that provides Chromium (though I cannot get the build to succeed).
The downside to all of these not being provided in mainline is the amount of time and energy to build them. I suspect there are more people than I would like curating their build of the linux kernel, manually specifying drivers included in the kernel. I really wish the community were a little less hard-set on the principle of 'freedom' and a little more pragmatic; I am sure the community is smaller due to people being turned away at the sheer amount of effort required to get their machine to a usable state.
[1]: https://github.com/wingo/guix-nonfree [2]: https://gitlab.com/mbakke/guix-chromium
They actually don't draw it exactly where I would. I don't care so much for games if they're open/free or not (it's nice when they are of course), but I am concerned with unauditable code on, for instance, the SATA controllers in my machine, even if it 'read only' code. Where the FSF is more concerned with the first and less with the second.
On the other topic, I think the ability to create (and share) your own recipes/channels for software on Guix allows for people to create fairly easy ways to install non-free software (or using Nix on top of GuixSD) without the Guix team having to support unethical software.
At the institute where I work we use this feature to provide package variants that aren't going to be added to Guix proper (e.g. because they are no longer supported upstream or because they are only useful on obscure systems).
This mechanism can be used to include package definitions for non-free software, of course, but this doesn't mean that the Guix project encourages the use of non-free software.
You have the freedom to package up and use non-free software, but we won't make them part of Guix, build them on our build farms, and we won't hold back changes to the mechanism even if that might break 3rd-party collections of non-free software.
You don't really need a blog post about how to set up a package repository, because it's right there in the Guix manual.
PS: It's "Guix", not "GUIX" ;)
Also, thank you for the capitalization correction. It grinds my gears when people capitalize things unnecessarily and I wouldn't want to be one of them!