You should be able to retrieve it with this little hack
cat << EOF > /etc/zypp/repos.d/base-15.5.repo
[base-15.5]
enabled=1
autorefresh=1
baseurl=http://download.opensuse.org/distribution/leap/15.5/repo/oss
keeppackages=0
priority=150
EOF
zypper refresh
zypper install grommunio-admin-api