Fuchsia is open source [1] [2] though not specifically GPL. It uses a myriad of licenses.
> I wonder how/if that will impact the platforms popularity.
It won't. Vast majority of users don't care about open source, and developers won't have to care. The kernel and userland get replaced, but the underlying Android OS (excluding GApps) remains open source.
With both the kernel and userland being replaced what, exactly, do you consider the Android OS to consist of?
I assume you're referring to the Java based frameworks and Dalvik/ART runtime pieces, but I would usually consider those part of userland, and your comment is a little unclear.
And Android can finally throw out its biggest chunk of meaningfully open-source source-code so far (from an end-user/modder perspective).
At that point Android is not going to be meaningfully open any more, and might as well be considered a closed platform on par with iOS.
I wonder how/if that will impact the platforms popularity.