I guess you're left with doing exec. Something like this should work:
exec { 'ruby193_scl_install_blah':
command = scl enable ruby193 'gem install --bindir /usr/bin --no-rdoc
--no-ri blah',
path= ['/bin','/usr/bin'],
unless = scl enable ruby193 'gem list'| grep -qs
From the silence, I'm going to guess that the gem provider cannot install
gems from Ruby installed from the Software Collections.
I did find a somewhat-related bug: '[PUP-3613] Add a parameter to the exec
resource to support running commands in software collections without always
using scl
