There have been "GNU" distributions that used non-Linux kernels. There was one that used the Solaris kernel, which at the time was even copylefted. Though with a license that was incompatible with GPL. (I think that Sun didn't want Linux just cutting and pasting stuff like ZFS and Containers into the Linux kernel.)

I'd also be surprised if there haven't been dists that used the GNU commands, but with a BSD kernel.