GNU is an short for GNU's Not Unix and is therefore a recursively-defined acronym.
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.