GNU a recursive acronym for GNU's Not Unix, project founded by Richard Stallman to promote free software.
GNU refers to various closely related things, but they all stem from Richard Stallman and the FSF's attempt to create a complete UNIX system that is completely free from commercial ties.
Tcl programmers should take note of two particular aspects of the GNU project:
gcc is called nowadays the GNU Compiler Collection, given the variety of front- and backends to the same internal engine.