  In order to improve Super-Kamiokande's neutron detection efficiency and to
thereby increase its sensitivity to the diffuse supernova neutrino background
flux, 13 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ (gadolinium sulfate
octahydrate) was dissolved into the detector's otherwise ultrapure water from
July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations.
During the loading, water was continuously recirculated at a rate of 60
m$^3$/h, extracting water from the top of the detector and mixing it with
concentrated $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ solution to create a 0.02%
solution of the Gd compound before injecting it into the bottom of the
detector. A clear boundary between the Gd-loaded and pure water was maintained
through the loading, enabling monitoring of the loading itself and the spatial
uniformity of the Gd concentration over the 35 days it took to reach the top of
the detector. During the subsequent commissioning the recirculation rate was
increased to 120 m$^3$/h, resulting in a constant and uniform distribution of
Gd throughout the detector and water transparency equivalent to that of
previous pure-water operation periods. Using an Am-Be neutron calibration
source the mean neutron capture time was measured to be $115\pm1$ $\mu$s, which
corresponds to a Gd concentration of $111\pm2$ ppm, as expected for this level
of Gd loading. This paper describes changes made to the water circulation
system for this detector upgrade, the Gd loading procedure, detector
commissioning, and the first neutron calibration measurements in SK-Gd.
