This is implies that Gentoo FreeBSD uses a GNU userland, which is incorrect. Gentoo FreeBSD uses a BSD userland. The quantity of GNU software present in Gentoo FreeBSD is comparable to the quantity of GNU software in FreeBSD itself.Gentoo FreeBSD is just like Debian GNU/kFreeBSD, except that it's built around Gentoo rather than pairing the Debian GNU user-land with the non-Linux kernel.
Gentoo itself is not tied to a particular userland and being sourced based makes it easy to give users a choice of userlands. We currently only offer one option per kernel (unless uclibc on Linux counts as a separate userland), but I would be thrilled to provide users with the ability to use a BSD userland on Linux. That likely will not happen because no one is working on that. The only thing close to that is an effort that I am making to port some FreeBSD components to Linux to improve ZFS support.