It's not about GNU.

V6 Unix (circa 1975) had it: http://man.cat-v.org/unix-6th/6/units

Your macOS have the BSD version:

    % units -v
    
      units version 1.0  Copyright (c) 1993 by Adrian Mariano
                        This program may be freely distributed
    usage: units [-f unitsfile] [-q] [-v] [from-unit to-unit]