For a while? Wasn't GNU Hurd supposed to be the kernel? Just vague memories, didn't read up on it right now. Still not even close to ready.
Linux is not a GNU project. But it's free software, so I suppose RMS could use it without major pain. Although he'd probably prefer Linux to use GPLv3.
GNU started with rewriting the Unix userland (especially GCC) years before announcing the Hurd effort. I think they developed and tested on BSD and SunOS, and later Linux (because they had nearly written the first distro, only without a kernel).
Linux is not a GNU project. But it's free software, so I suppose RMS could use it without major pain. Although he'd probably prefer Linux to use GPLv3.