For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/openSUSE_Tumbleweed/devel:languages:ruby.repo
zypper refresh
zypper install rubygem-gem2rpm
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/16.0/devel:languages:ruby.repo
zypper refresh
zypper install rubygem-gem2rpm
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/15.6/devel:languages:ruby.repo
zypper refresh
zypper install rubygem-gem2rpm