If you are on RHEL 7, you have to do this to get the RPMs:
subscription-manager repos --enable rhel-server-rhscl-7-rpms
I verified RHEL 7 has the “rh-ruby25” packages still in repos despite being EOL a long time ago.
https://osc.github.io/ood-documentation/release-1.7/installation/install-software.html
Verified that CentOS 7 still has the packages too so most likely you are just missing the repos you need to provide the SCL packages.