I doubt that the primary motivation for Magenta is to eliminate the GPL'd Linux kernel from Android. A much more practical way to do that would be to use one of the BSDs.

They also deprecated GCC from the NDK, going forward only clang will be supported.

GCC is still around, because just like it happened with Apple, there are a few features that clang still lacks in order to fully replace it in the context of Android.

Brillo has even less GPL components than Android.

I don't think you can legitimately say that Google are hampering open source by releasing more open source software. You might argue that they're stepping back from the GPL, but that's different.

I bet in around 10 years time, the GPL will be surely missed, but then it will be too late.

+1 Even today Android is falsely advertised as open source while in fact you can't install the OSS version on any phone and expect it to work without proprietary blobs. Thank closed device drivers for that, and more restrictions will come in the future.

Right now the OSS community should focus their efforts on the most important goal: having fully open source hardware CPUs and peripherals. We already have huge loads of open source software but we still lack a comparatively open iron to run them on.

Well, RISC-V is coming and some of that will be fully open source, freely licensed hardware. For example: http://www.lowrisc.org/

Peripherals are a lot more tricky. RISC-V is concentrating on the CPU cores, cache hierarchy and interrupt controller. Peripherals will be proprietary for a while, but could be open source one day.

Out of curiosity, will this end custom ROMs?

Not necessarily, but it will be harder work reverse-engineering what OEMs do without the modified kernel source releases