For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mschnitzer:ruby:extensions/openSUSE_Tumbleweed/home:mschnitzer:ruby:extensions.repo
zypper refresh
zypper install rubygem-font_awesome5_rails
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mschnitzer:ruby:extensions/15.4/home:mschnitzer:ruby:extensions.repo
zypper refresh
zypper install rubygem-font_awesome5_rails
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mschnitzer:ruby:extensions/15.3/home:mschnitzer:ruby:extensions.repo
zypper refresh
zypper install rubygem-font_awesome5_rails