rubygem-gem2rpm from home:Leen-Meyer:devel:languages:ruby project
Select Your Operating System
Add repository and install manually
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Leen-Meyer:devel:languages:ruby/15.6/home:Leen-Meyer:devel:languages:ruby.repo zypper refresh zypper install rubygem-gem2rpm