The main rubygen-rack package is not in the PackageHub repositories.
Many of its subpackages are though, but they cannot be installed without the
main package.

Problem: nothing provides 'rubygem(ruby:2.5.0:rack) >= 0' needed by the to be
installed ruby2.5-rubygem-rack-protection-2.0.1-bp153.1.16.x86_64
 Solution 1: do not install
ruby2.5-rubygem-rack-protection-2.0.1-bp153.1.16.x86_64
 Solution 2: break ruby2.5-rubygem-rack-protection-2.0.1-bp153.1.16.x86_64 by
ignoring some of its dependencies

It does seem the rubygem-rack package is within the SUSE Linux Enterprise High
Availability Extension for 15 SP3.

We need the rubygem-rack also in PackageHub with the rest.
The package was available in PackageHub for SLES 15 SP2.