  The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m
diameter mirror segments that are separated by large > 30 cm gaps, creating the
possibility of fluctuations in optical path differences due to flexure, segment
vibrations, wind buffeting, temperature effects, and atmospheric seeing. In
order to utilize the full diffraction-limited aperture of the GMT for natural
guide star adaptive optics (NGSAO) 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, an off-axis dispersed fringe
sensor (part of the AGWS), and a pyramid wavefront sensor (PyWFS; part of the
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 working prototype of a "GMT High-Contrast Adaptive
Optics phasing Testbed" (p-HCAT) which leverages the existing MagAO-X AO
instrument to demonstrate segment phase sensing and simultaneous AO-control for
GMT NGSAO science. We present the first test results of closed-loop piston
control with one GMT segment using MagAO-X's PyWFS and a novel Holographic
Dispersed Fringe Sensor (HDFS) with and without simulated atmospheric
turbulence. We show that the PyWFS alone was unsuccessful at controlling
segment piston with generated ~ 0.6 arcsec and ~ 1.2 arcsec seeing turbulence
due to non-linear modal cross-talk and poor pixel sampling of the segment gaps
on the PyWFS detector. We report the success of an alternate solution to
control piston using the novel 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 piston to within 50 nm RMS and $\pm$ 10 $\mu$m
dynamic range under simulated 0.6 arcsec atmospheric seeing conditions.
