The Jargon File (version 4.4.7, 29 Dec 2003):

GNUMACS
 /gnoo'maks/, n.

    [contraction of ?GNU EMACS?] Often-heard abbreviated name for the GNU
    project's flagship tool, EMACS. StallMACS, referring to Richard Stallman,
    is less common but also heard. Used esp. in contrast with GOSMACS and X
    Emacs.


The Free On-line Dictionary of Computing (30 December 2018):

GNUMACS

   /gnoo'maks/ [contraction of "GNU Emacs"] Often-heard
   abbreviated name for the GNU project's flagship tool,
   Emacs.  Used especially in contrast with GOSMACS.

   [Jargon File]