With its inaugural Antarctic long-duration balloon mission in December 2023, the General An-
tiparticle Spectrometer (GAPS) will become the first experiment optimized to detect cosmic-ray
antinuclei below 0.25 GeV/<i>n</i>. Detection of a single antideuteron at this energy scale would
be a smoking-gun signature of new physics such as dark matter. The GAPS program will also
provide a precision antiproton spectrum in a previously unprobed low-energy range, as well as
leading sensitivity to antihelium-3. This new parameter space is accessible thanks to a novel par-
ticle identification method based on exotic atom formation, de-excitation, and decay. The method
provides a unique handle for the negatively-charged antinuclei, facilitating excellent rejection of
the positive-nucleus background, and does not require a magnet, enabling a large sensitive area
for rare events. The GAPS instrument is designed to provide excellent discrimination power for
rare events within the power and mass constraints of a long-duration balloon. The time-of-flight,
composed of 160 scintillator paddles, provides the system trigger as well as the GAPS energy
scale. The 2.5 m3 tracker volume is instrumented with 1000 10-cm-diameter silicon sensors which
serve as the target, X-ray spectrometer, and particle tracker. Together, a large-area radiator and an
integrated oscillating heat pipe system cool the payload without a bulky cryostat. This contribution
reports the integration and calibration of the GAPS science payload, including the performance of
the sensitive detector subsystems, the cooling system, the power distribution, and data acquisition
and onboard event processing.