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.
> 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.
[1] https://github.com/fuchsia-mirror
[2] https://en.wikipedia.org/wiki/Google_Fuchsia