if !gem_installed("some gem name")
  system "gem install -l local_copy.gem"
end

begin
  gem "somegem"
  # with requirements
  gem "somegem", ">=2.0"
rescue GEM::LoadError
  # not installed
end

matches = Gem.source_index.find_name(gem.name, gem.version_requirements)

puts %x(#{cmd})