We describe the conceptual optomechanical design for GMACS, a wide-field, multi-object, moderate-resolution optical
spectrograph for the Giant Magellan Telescope (GMT). GMACS is a candidate first-light instrument for the GMT and
will be one of several instruments housed in the Gregorian Instrument Rotator (GIR) located at the Gregorian focus. The
instrument samples a 9 arcminute x 18 arcminute field of view providing two resolution modes (i.e, low resolution, R ~
2000, and moderate resolution, R ~ 4000) over a 3700 Å to 10200 Å wavelength range. To minimize the size of the
optics, four fold mirrors at the GMT focal plane redirect the full field into four individual "arms", that each comprises a
double spectrograph with a red and blue channel. Hence, each arm samples a 4.5 arcminute x 9 arcminute field of view.
The optical layout naturally leads to three separate optomechanical assemblies: a focal plane assembly, and two identical
optics modules. The focal plane assembly contains the last element of the telescope's wide-field corrector, slit-mask,
tent-mirror assembly, and slit-mask magazine. Each of the two optics modules supports two of the four instrument arms
and houses the aft-optics (i.e. collimators, dichroics, gratings, and cameras). A grating exchange mechanism, and
articulated gratings and cameras facilitate multiple resolution modes. In this paper we describe the details of the
GMACS optomechanical design, including the requirements and considerations leading to the design, mechanism
details, optics mounts, and predicted flexure performance.