The userland is quite GNU. Think bash, gawk and coreutils (ls, etc.) as well as the libraries the userland depends on -- including the C and C++ standard libr

The userland is quite GNU. Think bash, gawk and coreutils (ls, etc.) as well as the libraries the userland depends on -- including the C and C++ standard libraries, pthreads, readline, etc. Very little of it is irreplaceable these days. You could use a different shell. You can even run a BSD userland under a Linux kernel, in theory. The Linux kernel itself is replaceable with e.g. the open sourced Solaris kernel. There's many alternatives to just about any major UNIX component these days. But the standard (and often the only fully supported) set of tools and libraries on Linux is heavily GNU.