A quick peek into ruby/bin/gem revealed much of what I needed to know.

Lets demonstrate :

Welcome to the JRuby IRB Console 

irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> Gem.manage_gems
=> true
irb(main):003:0> gem = Gem::GemRunner.new
=> #
irb(main):004:0> gem.run(['help','commands'])
GEM commands are:

    build            Build a gem from a gemspec
    cert             Adjust RubyGems certificate settings
    check            Check installed gems
    ...
=> nil
irb(main):005:0>

update: The bug that prevented me from installing gems, turned out to be a _gemrc file that had http_proxy configured. A proxy no longer in use, but since I didn’t use gems all that much on this workstation before – I forgot all about it.