  gPhoton is a new database product and software package that enables analysis
of GALEX ultraviolet data at the photon level. The project's stand-alone,
pure-Python calibration pipeline reproduces the functionality of the original
mission pipeline to reduce raw spacecraft data to lists of time-tagged,
sky-projected photons, which are then hosted in a publicly available database
by the Mikulski Archive at Space Telescope (MAST). This database contains
approximately 130 terabytes of data describing approximately 1.1 trillion
sky-projected events with a timestamp resolution of five milliseconds. A
handful of Python and command line modules serve as a front-end to interact
with the database and to generate calibrated light curves and images from the
photon-level data at user-defined temporal and spatial scales. The gPhoton
software and source code are in active development and publicly available under
a permissive license. We describe the motivation, design, and implementation of
the calibration pipeline, database, and tools, with emphasis on divergence from
prior work, as well as challenges created by the large data volume. We
summarize the astrometric and photometric performance of gPhoton relative to
the original mission pipeline. For a brief example of short time domain science
capabilities enabled by gPhoton, we show new flares from the known M dwarf
flare star CR Draconis. The gPhoton software has permanent object identifiers
with the ASCL (ascl:1603.004) and DOI (doi:10.17909/T9CC7G). This paper
describes the software as of version v1.27.2.
