But not just "linux" either. Same as we don't call Mac OS X "Darwin". Linux is just a kernel.
From
http://www.gnustep.org/:
GNUstep currently supports Unix (GNU/Linux and GNU/HURD, Solaris, NetBSD, OpenBSD, FreeBSD, Darwin) and Windows.
Looks like we do call Mac OS X "Darwin", or at least GNU does.