The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirrors, together forming a single 25.4-m diameter primary mirror. This large aperture and collecting area can help extreme adaptive optics (ExAO) systems such as GMT’s GMagAO-X 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 primary mirror segments are separated by large >30  cm gaps, creating the possibility of fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. To utilize the full diffraction-limited aperture of the GMT for high-contrast, natural guide star-adaptive optics science, 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 slow (∼0.03  Hz) off-axis dispersed fringe sensor (part of the acquisition guiding and wavefront sensing system’s active optics off-axis guider), and a pyramid wavefront sensor [PyWFS; part of the natural guide star wavefront sensor (NGWS) adaptive optics] 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 working prototype of a “GMT high contrast adaptive optics phasing testbed” that leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT natural guide star science [i.e., testing the NGWS wavefront sensor (WFS) architecture]. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS with and without simulated atmospheric turbulence. We show that the PyWFS was able to successfully control segment piston without turbulence within 12- to 33-nm RMS for 0 λ  /  D to 5 λ  /  D modulation, but was unsuccessful at controlling segment piston with generated ∼0.6  arcsec (median seeing conditions at the GMT site) and ∼1.2  arcsec seeing turbulence due to nonlinear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT (and likely the TMT and ELT). Hence, a dedicated “second channel” piston sensor is required. We report the success of an alternate solution to control piston using a holographic dispersed fringe sensor (HDFS) while controlling all other modes with the PyWFS purely as a slope sensor (piston mode removed). This “second channel” WFS method worked well to control segment piston to within 50 nm RMS and ±10  μm dynamic range under simulated 0.6 arcsec atmospheric seeing (median seeing conditions at the GMT site). These results led to the inclusion of the HDFS as the official second channel piston sensor for the GMT NGWS WFS. This HDFS + PyWFS architecture should also work well to control piston petal modes on the ELT and TMT telescopes.