The Giant Magellan Telescope design consists of seven circular 8.4 m diameter mirrors, together forming a single 24.5 m diameter primary mirror. This large aperture and collecting area can help extreme adaptive optics systems such as GMagAOX achieve the small angular resolutions and contrasts required to image habitable zone earth-like planets around late type stars and possibly lead to the discovery of life outside of our solar system. However, the GMT mirror segments are separated by large ? 30 cm gaps, creating the possibility of fluctuations in optical path differences (piston) due to flexure, wind loading, temperature effects, and atmospheric seeing. In order to utilize the full diffraction-limited aperture of the GMT for high-contrast imaging, the seven mirror segments must be co-phased to well within a fraction of a wavelength. The current design of the GMT involves seven adaptive secondary mirrors, a dispersed fringe sensor (part of the AGWS), and a pyramid wavefront sensor (NGWS) to measure and correct the total path length between segment pairs, but these methods have yet to be tested "end-to-end"in a lab environment. We present the design and prototype of a "GMT High-Contrast Phasing Testbed"which leverages the existing MagAO-X ExAO instrument to demonstrate fine phase sensing and simultaneous AO-control for high-contrast GMT natural guide star science. The testbed will simulate the GMT primary and secondary mirror phasing system. It will also simulate the future GMT ExAO instrument's (GMagAO-X) "parallel DM"tweeter concept of splitting up the GMT pupil onto several commercial DMs using a reflective hexagonal pyramid. A dispersed fringe sensor will also be implemented into the testbed for coarse piston phase-sensing along with MagAO-X's pyramid wavefront sensor to measure and correct the fine phasing level of the GMT primary mirror segments under realistic wind load and seeing conditions.