  This paper reports the development and detailed properties of about 13 tons
of gadolinium sulfate octahydrate, $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$, which
has been dissolved into Super-Kamiokande (SK) in the summer of 2020. We
evaluate the impact of radioactive impurities in $\rm Gd_2(\rm SO_4)_3\cdot \rm
8H_2O$ on DSNB searches and solar neutrino observation and confirm the need to
reduce radioactive and fluorescent impurities by about three orders of
magnitude from commercially available high-purity $\rm Gd_2(\rm SO_4)_3\cdot
\rm 8H_2O$. In order to produce ultra-high-purity $\rm Gd_2(\rm SO_4)_3\cdot
\rm 8H_2O$, we have developed a method to remove impurities from gadolinium
oxide, Gd$_2$O$_3$, consisting of acid dissolution, solvent extraction, and pH
control processes, followed by a high-purity sulfation process. All of the
produced ultra-high-purity $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ is assayed by
ICP-MS and HPGe detectors to evaluate its quality. Because of the long
measurement time of HPGe detectors, we have employed several underground
laboratories for making parallel measurements including LSC in Spain, Boulby in
the UK, and Kamioka in Japan. In the first half of production, the measured
batch purities were found to be consistent with the specifications. However,in
the latter half, the $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ contained one order
of magnitude more $^{228}$Ra than the budgeted mean contamination. This was
correlated with the corresponding characteristics of the raw material
Gd$_2$O$_3$, in which an intrinsically large contamination was present. Based
on their modest impact on SK physics, they were nevertheless introduced into
the detector. To reduce $^{228}$Ra for the next stage of Gd loading to SK, a
new process has been successfully establised.
